So converting characters is easy, but how to convert monsters? This was a big issue for me because I run my games on Eberron, and a lot of the material was written for 3e. It was particularly difficult because I never played 3e (I skipped from a heavily houseruled mix of 1e and 2e straight to 4e).
I decided to figure it out in terms of how powerful the monsters were in relation to the characters, or the character party.
Converting Third Edition Monsters
From my limited understanding of 3e (and someone can fell free to correct me on any of this if I am wrong), monster strength is measured in CR, with the intention that a monster of CR n is a moderate challenge for a party of four characters of level n. Sort of like an at-level 4e solo monster, but intended to fight four characters instead of five.
However, I read in various commentaries, monsters of CR n usually were not much of a challenge to parties of level n on their own. And then I later read that monsters that were created using the rules for building characters were usually much less challenging than monsters that were created using the rules for building monsters. Which made sense to me, since I had already figured out that a single NPC of level n was unlikely to be much of a challenge for a party of four PCs built identically to the NPC. Although if he was going all out he might not go down easily.
In 4e a number of standard monsters of level n is a moderate challenge for like number of PCs of level n. But if those PCs fight the same number of elite monsters of level n, it can be touch and go. As a rule, when used in a group, an elite monster is pretty much equivalent to a PC of the same level (the same is not true of one-on-one fights, but we will save that for another discussion).
So if I am converting an NPC from 3e to 4e, I first figure out the equivalent level, and then build the monster as an elite NPC of that level. So a 3e third level fighter would be converted to a sixth level elite monster. And a seventh level fighter would be converted to a twelfth level elite monster.
For fractional CRs, I have been dividing the 4e XPs by the fraction to figure out the appropriate monster level. So if a CR 1 NPC is a level 2 elite worth 250 XPs, then a CR 1/2 monster is a level 2 standard worth 125 XPs.
This would make 3e orc and hobgoblin warriors the equivalent of a level 2 standard monster; a 3e goblin warrior is the equivalent of a level 8 minion (or a level 4 half-standard monster – see how they come in handy?); and a 3e kobold warrior is the equivalent of a level 6 minion (or a level 2 half-standard monster).
I don’t know enough about 3e to assess how tough monsters built as monsters are, but I am prepared to treat them as being as tough as they were intended to be, that is, a CR n monster built as a monster is a moderate threat for a party of four level n characters. After you convert 3e level to 4e level, that roughly works out to be a solo monster of 4e level n-1. Sort of, depending on how you build your solos.
So a third level 3e monster would be converted to a fifth level solo monster. And a seventh level 3e monster would be converted to an eleventh level solo monster.
Converting First Edition Monsters
I also convert a lot of first edition modules, and I have been trying to build 1e-style encounter tables for use in 4e. This has been a bit of a challenge because first edition monster level is only vaguely related to its level of challenge, being more an indication of where they are likely to appear in a dungeon.
I have settled on experience point value as the best way of determining monster challenge relative to the PCs. A first level fighter is worth 20 + 2/hp XPs (or 28 + 2/hp XPs if he has a missile weapon). This is identical to a 1e hobgoblin, which stands at the top of the range of level I monsters; and is about twice the power of a 1e orc, which is worth 10 + 1/hp experience and is in the mid-range of level I monsters.
In 1e, the experience necessary to level more or less doubles every level until you hit “name” level (usually at around tenth level). A similar progression applies to monster levels, so that a level II monster is worth more or less twice as much as a level I monster, and a level III monster is worth more or less twice as much as a level II monster. Sort of. Until you get to level X monsters, of which the lowest is worth 10,001 XPs, and the highest (Demogorgon) is worth 74,000 (or 740,000 if killed on his own plane).
So you might use monster level as a proxy for character level, at least until level X, when you have to figure something else out. So a hobgoblin, which is at the high end of level I, is the equivalent of a level 1 1e character, which we can convert to a level 2 elite monster (or a level 6 standard monster). A gnoll is a level II monster, which we can convert to a level 4 elite monster (or a level 8 standard monster). A bugbear or an ogre is a level III monster, which we can convert to a level 6 elite monster (or a level 10 standard monster).
For monsters on the weaker end of level I we divide the 4e XPs by the fraction of experience the monster is worth in relation to the experience of a hobgoblin. So if a hobgoblin is worth 20 + 2/hp XPs, and an orc or goblin is worth 10 + 1/hp XPs, and a kobold is worth 5 + 1/hp XPs., then a converted orc or goblin is worth half as much as a converted hobgoblin, and a converted kobold is worth a quarter as much. That means an orc or goblin would be a level 2 standard monster, and a kobold would be a level 6 minion (or level 2 half-standard monster).
For monsters of level X, in keeping with the experience required by PCs to increase in level, monster level is increased by 1 for each 5,000 XP over 10,000. So an aspect of Demogorgon would effectively be level XXI, or perhaps a 4e level 26 elite monster (although all the conversions tend to break down after level 20).
Converting Fifth Edition Monsters
I’ve been looking at 5e as a game for when I want to play on a tabletop without computers. Because I will admit 4e combat is kind of a pig if condition-tracking isn’t computer-assisted. So I have been thinking about monster equivalencies there as well, and for this conversion I am also thinking about how challenging the monsters are for a given level of PC.
For the purposes of estimating challenge, the XP value of a monster increases when there are greater numbers of them. So we need to incorporate that XP budgeting system into how we evaluate monsters. And we need to think about the numbers that a monster typically comes in when evaluating it, because monsters that typically function in numbers, like goblins and orcs, will convert differently from monsters that typically work alone, like dragons (and FYI, I’m not ready to think about legendary monsters yet).
For a group of five level 1 PCs, a deadly encounter would be worth 500 XPs. This is the equivalent of five CR 1/4 monsters. Mapping this to 4e, this means a CR 1/4 monster, like a goblin, is the equivalent of a 4e level 2 elite (level 6 standard) monster. That’s a pretty tough goblin. One-on-one it would be an easier battle, the equivalent of a level 2 standard; but we don’t generally care how many goblins an individual PC can take, we are concerned with how many a PC party can take.
Following that pattern, a group of CR 1/8 monsters can be treated as level 2 standard monsters; a group of CR 0 monsters can be treated as level 6 minions (level 2 half-standard monsters); a group of CR 1/2 monsters can be treated as level 4 elite (level 8 standard) monsters; and a group of CR 1 monsters can be treated as level 6 elite (level 10 standard) monsters.
Which is fine in theory. But suggesting that a 5e goblin is equivalent on power to a 1e hobgoblin just seems wrong to me. I suspect I am going to have to learn more about the actual power of 5e monsters in play before I can make meaningful conversions.