Converting monsters across editions of Dungeons & Dragons can be quick and easy. It is so easy that I do it at the table on the fly. I do not find it necessary to figure out before game. The best way to do it is to understand what is going on with the monster stats, especially the Hit Points and Armor Class. I run my game often with Castles & Crusades, I consider it the Rosetta stone of D&D. It takes the third edition rule set and brings it back from the more complex elements and delivers a base game that is easy to convert back and forth with other editions. It is a great game to see where the changes occur between editions. It has an ascending armor class, which means the AC number gets higher and the players must roll equal to or over their opponents’ AC to hit. This means a higher AC was better and makes the monster or PC harder to hit. Editions before third had a descending AC meaning the lower the AC number the harder to hit they became. This was figured out a couple ways depending on the edition or rules you were using for the edition. To Hit Armor Class Zero (THAC0) is often brought up here. It might be interesting to note that THAC0 Was a newer element to the game which became an official rule in the late 80s with the release of 2nd edition AD&D. The editions prior to this used charts. The charts were easy for players because only the Dungeon Master had to pay attention to them. They made combat more of a mystery. Often players were not looking at the number they had to roll over but rolling and telling the DM to see if they hit the monster. The DM would look at the AC of the monster and the level and class of the PC and determine by the chart what had to be rolled to hit the monster. I personally find this the easiest of the two descending AC mechanics.
For the most part, the formula is simple in taking a descending AC stat to a game like Castles & Crusades. You subtract the existing AC number from 20, (sometimes this is 21 or 19 depending on what you are converting for where) this inverts the number giving you a new AC. You go with the initial hit die, and just roll them up. Viola, there you have it. That is the easy quick way to do it. You can get dirty with it if you want, and really get into understanding what is going on and that really does help in doing this in game. We are going to look at this. In the end, the best way for you to convert monsters across editions and versions is to spend time comparing Monster Manuals from the two versions you are using. This works with games based on the Dungeons & Dragons mechanics and the OGL or earlier. The newer the edition, the less this works.
I think the ascending AC is a better mechanic. I played D&D for quite a while before the ascending AC was released with 3rd edition in the year 2000. As someone who came to D&D through other Role Playing Games, I was introduced to forms of ascending AC years before it was in D&D, and I thought it was just a simpler streamlined idea. I only ran one 3rd edition campaign in 2000, when it first came out. The issues that I had with the D&D 3rd edition were not with the AC, I thought that was a great idea. Castles & Crusades also uses this with the same basic ratio and formula that is derived from earlier editions. I might note that the 5th edition completely threw away this ratio from all the previous editions making your older books pretty much useless for conversion. The AC of 5th edition Monsters tend to be much lower than the previous editions, and the HP tend to be much higher. This change makes it much easier to hit monsters, but they must take more damage, more hits to kill. I have yet to figure out any correlation between the stats of a 5th edition D&D monster and those of previous editions. I am not even touching 4th edition, because I have never played it. Nothing against it, it was just short lived and I never picked it up. I also never picked up 5th edition, my only experience was playing in a few games with a group when it first came out, and I did not DM. So, we will be looking at newer retro-clones, Pathfinder, 3.5 and earlier editions of D&D. Which is most D&D editions and versions. Also, the D&D that was played the longest.
AC in general in 3.5, 3rd, Pathfinder and C&C starts at a base of 10. An unarmored human has a base AC of 10. Which means, anyone trying to hit that person or monster needs to roll a 10 or higher to hit that creature. Before that in 2nd edition with the use of THAC0 the base AC was also 10, only it went down as the character gained better armor, so to determine the number needed to hit you would take the number given by level as what was needed to hit AC 0 and subtract from that number. Let us say you have a THAC0 of 20, and you are going against a monster with an AC of 5. Subtract the 5 from the 20, and you need a 15 or higher to hit. As the character gains levels the THAC0 decreases making the opponent easier to hit. In the family of 3rd edition versions where we see the ascending AC this is reflected in a bonus to hit. Instead of adjusting the formula to hit, the bonus is given and added to the dice roll and not subtracted. The THAC0 number given by level and class more accurately reflects the math from the AD&D and OD&D to hit charts. This made most all the earlier editions compatible. The notable difference was that monsters in general were one point lower in Basic versions of the game as compared to AD&D. This means instead of a base of 10, we might be looking at a base of 9. The monsters would shift one point down in AC. If you pull a monster from the Rules Cyclopedia or the Basic Expert set, you will need to add one point to all AC if converting to AD&D. They need to be one point harder to hit.
The best way possible to figure out conversions between editions is to take the editions and compare them. It is easier to convert from 3rd and later to fit your older editions or to fit your C&C game than it is to convert from an older edition to 3rd and later. If you look at books of 3rd, 3.5 or Pathfinder you find the excessive statistics not in older editions or editions such as C&C. So, this is primarily for converting back to older editions and retro clones. Older editions had a stat for hit die, it was normally just one number. You would roll one D8 for every number noted as your hit die. So, a monster with 5 hit die, would have 5D8 hit points. But, when you get to 3rd and later, they start handing out bonuses to hit, skills, feats, and other things that began a power creep in the game like never before. To reflect this creep, hit die were no longer a single number to reflect this, but it would list 5D8 +12. The hit die was getting padded to reflect the power creep of player characters. The hit die ceased to be a D8 and began to become just any die number you could come up with. If you look at the ratio though, the stat block could be taken back by simply dropping everything after the initial Hit Die number. For example, now a monster might have 5D10 + 8 for their hit die. Whereas in an earlier edition just the 5 was needed and everything else after was not. So, in this case we are only looking to use the initial number, the 5. The monster has 5 Hit Die, and you can just stick with the D8 hit die and not worry about the other stats given in the Monster Manual. As a house rule, I still use D8’s for hit dice. I will stray a little depending on the given monster, but since I do a lot of converting, it is just way easier to keep the D8 as the hit dice. You can deviate from that as much or little as possible if you want.
Let us compare some monsters between editions and illustrate this. When using common monsters like the Ogre, or the ones you will be comparing in the Monsters Manuals, it is advisable to just use the one from the system you are running. This is just a method I use to port in monsters that were not made for a specific system. The other time I use this is when the stats for monsters are in a module and I am trying to reduce clutter by using them instead of getting out another book. The Ogre in C&C has an AC of 16 and 4D8 HP. The Ogre in AD&D has an AC of 5 and Hit Dice of 4+1 (now that’s a +1 for each hit die so whatever 4D8 is rolled plus four). Let’s look at Swords & Wizardry. S&W is a great resource for looking at conversions because it gives both the ascending as well as descending AC of each monster in it’s stat block. The Ogre has an AC of 5  (the first number being the descending AC and the number in brackets  is the ascending AC), and the ogre in S&W gets hit points of 4+1. Now it is worth noting that the ascending AC in C&C tends to run a little high compared to the ascending AC in C&C. So we see a full two point difference here. The AC between the AD&D and S&W are spot on though for this one. It is worth noting that the descending to ascending from S&W seems to be best achieved by subtracting from 19. The good part is that you do not have to subtract with. To get the AC for C&C from AD&D we see that there is a 21 point invert to achieve this. We see the AD&D is 5 (like the S&W version) subtract that from 21 you get 16. But wait! There is more!
This is not a universal number you figure out to plug in and convert directly. There is a lot of judgment calls to be made. Both the AD&D and the S&W have Hit Points of +1 but we do not see that with the C&C stat block. The C&C stat block also has the highest Ogre AC, so this call as to pull in the +1 for AC or not is totally yours to make. I would determine this based on what I know about how easily my PC’s could take this guy. Often the extra +4 is minimal. Want a harder monster, go with it. Want it easier? Leave it off. Now, this is intended to be used for monsters not in the Monster Manual you are using for the system you are playing, so you will have to make the call off of what you see for a monster that is not represented in the system you are playing for comparison. If you look at Pathfinder The AC shoots up to a 17, and the hit dice are suddenly 4D8 +12. Wow! That went up quick! If we look at the Ogre in 3rd edition (Not 3.5 but third) we see the AC is around the C&C with a 16, and the Hit Die is a 4D8 +8. You can see the progressive power creep from the AD&D +1 to the 3rd edition +8 and the Pathfinder +12. Because of this I tend to drop anything after the 4D8 when converting backwards from newer editions. The book I have done this with most is my copy of The Tome Of Horrors for Pathfinder when converting to Castles & Crusades. Converting from Pathfinder to AD&D or Basic you would still invert by subtracting from a number like 19, 20, or 21, but with such a power bloat, I would almost look at subtracting from 22 to bring a monster from pathfinder to AD&D.
My current campaign is a C&C campaign ran in The Lost Lands. The Lost Lands books were initially released under the D20 system, which is the 3rd edition and 3.5, and later books in the setting were Predominantly Pathfinder or Swords & Wizardry and 5th edition. I buy a lot of the S&W editions of Lost Lands books because I find them easiest to work from, but I often find I am using a 3rd edition or Pathfinder version of a book. So, I am going to do some converting and comparing using those systems to further delve into this. One of the books I have converted from the most is The Tome of Horrors Complete, which is available in Pathfinder or S&W. You can get print to order versions of the S&W edition on Amazon, and I cannot tell you how great this is as a Monster Manual for your game. Tons of new and unique different horrors for your players to encounter.
Now Orcus The Demon Prince of the Undead was in the AD&D Monster Manual, but never made it to the 2nd Edition Monstrous Manual because of the satanic panic and his being a Demon Lord. In The Tome of Horrors Complete for S&W, Orcus has the AC of -5 and his Hit Dice is 30. That means 30D8. Not the easiest thing to take out in the game. Pretty damn near impossible to hit with an AC like that. Remember, a negative AC in descending AC combat is good. The Pathfinder version of Orcus blows up huge, we are looking at an AC of 55, and Hit Points of 45D10 + 495. The power creep is really seen at higher levels here in Pathfinder and newer editions. For this reason you really need to have a good grasp on the system you are running. As higher levels go up in things like Pathfinder you see damage amounts not possible in previous editions. But wait! Orcus is in C&C in the Tome of the Unclean specifically by Troll Lord Games. And here Orcus has an AC of 26, so just two more than the ascending AC from S&W. It calculates out perfectly if you calculate using 21 from S&W to C&C with a negative 5 subtracting from the 21 putting you at the 26. (-5-21=26) If we look at the Ogre who had an AC of 5 and do the same (5-21=16) we see the same calculation that worked to bring the Ogre from S&W or AD&D (Because the two were the same) works to give us the C&C AC. This also shows just how similar C&C is to earlier editions as opposed to later editions despite its use of ascending AC.
There is a lot more ground to be covered between the varying editions and conversions, but this one is getting long, and I have plans to follow up making this just the first in a series of articles on conversion from varying editions of Dungeons & Dragons. I would love to hear feedback or observations from others who convert often. Also, if you want to have a version addressed shoot me a line and let me know, I can try to focus on those ones specifically. With so many versions the combinations are pretty vast in converting from edition to version.