Converting Monsters Old School -> New

It should be pretty clear that I love the aesthetic of Old School, and view the D20 system like a butcher views a fresh carcass; as something to be cut apart and rendered into a useful product.

But as much as I despise the d20 system, I still play D&D with it for the simple reason that it has the sense to unify the whole system under one mechanic: Roll a twenty-sider. Add the modifiers. Compare result to difficulty class of the roll.
It's fairlyy boggling how OS D&D gets praised for its "simplicity" When it's pretty plain that the rules were a mess.

As a result, I prefer to use the AD&D version of the MM and Fiend Folio, and convert them to something usable in d20.

Here's my process for converting Old School to d20 stats. 

Here is an entry from my beloved AD&D MM. This particular MM gives lots of environmental and behavioral info, and describes what parts of monsters are good for eating and selling and making into potions and all that. Important stuff since a monster is an idea first and a statblock last.

 ARMOR CLASS: AC worked differently in AD&D. It was on a scale from 10 to -10; 10 being unarmored and -10 being the arbitrary upper limit on armor. But the adjustments for armor stepped in the same way. Leather still improved AC by two, Chainmail by 6 and so on, so it can be rescaled.
To convert this to d20 / 3E, Multiply the old school AC by (-1) and add the result to 20.

(-1) (AD&D AC) + 20 = (d20 AC)

HIT DICE: Still the same. Except in 3E, the die used is determined by monster type, while in older editions, a hit die for monsters was always a d8, or a d6 in the original D&D.

THAC0: is an acronym for To Hit Armor Class Zero. It was the AD&D equivalent for base attack value. It literally meant the number needed in order to roll in order to hit someone with armor equivalent to full plate and a shield. A low THAC0 was better than a high one. The targets Armor Class was subtracted from the Thacko (that's how the acromyn was pronounced aloud) to yield the necessary to-hit.  Yeah. That's THAC0. It's hardly elegant. A THAC0 of 20 is the same as having a base attack bonus of 0, 19 is +1 and so forth.

(THAC0 - 20) (-1) = d20 base attack bonus,

or rather,
(20 - THAC0) = d20 base attack bonus, remembering extra attacks as applicable

MOVEMENT: Movement rates in old-schoolish are given as a number which is always a multiple of 3 for some reason. It is sometimes listed with the " inches symbol and sometimes with no unit at all. It is all a little cloudy. In AD&D, a combat round was about a minute long, and a character's full movement distance while hustling or walking was 10x the listed movement rate in yards. Base movement for a human or dwarf was given as 12', while shorties like dwarves and halflings are 6', or sometimes 9' So let's go with the 9. I guess the best way to convert to 3E movement rate is to subtract 3 from the rating, then divide by three, and multiply by 10 to find the base movement in feet.  If a monsters movement is 3, then obviously, there's no need to subtract the 3 from it. The monster is still allowed to move, even after edition conversion.

[ ( Oldschoolish movement - 3 ) / 3 ] times [ 10 ] = 3E base movement

XP VALUE: While AD&D gives straight up numbers for a monsters xp value, experience in 3E goes off a monsters "Challenge Rating." (challenge rating needs to go!) Until then, a monster's HD suffices as an equivalent to CR.

Now here is the same monster printed in the Fiend Folio. Like the above-mentioned MM, FF is AD&D. Except the MM is the 2nd edition of AD&D. Makes perfect sense, right? Anyways, the FF Grell is  pretty much the same as the MM. Except notably absent is the THAC0, which is kind of vital information. Imagine how mystified I was, about 13 and trying to learn about the hobby with little more to go an than these half-edited books with their ill-conceived, rules. 

...Stop helping, Dog.

Without listing the Thacko, the Fiend Folio is just a freaky-ass picture book. Lots of OSR homebrewers continue to stat monsters without listing any thacko or attack bonuses. Eventually, I did find some homebrewed tabled on OSR blogs which equated HD to combat abilities. That was useful.

But thankfully, I did find some "canon" attack tables in the B/X rules:

It's not in Thacko, but the same logic shows through in this table. But WHY IN THE NAME OF GYGAX'S GHOST IS AC RANGED FROM NINE TO NEGATIVE THREE!? Did Moldvay and Metzer ever hear of using elegant math? Why oh why couldn't they have started with a nice round number like 10? Its a mystery, clouded by the mist of the late 70s and early 80s.
Fortunately, they fixed this oversight in AD&D. There is a zero-column in the Monster Attack table, but the real Thacko is represented  under the -1 column

Attack bonuses, based on HD  

OS monster HD	less than 1	1	2	3	4	5	6	7
AD&D THACO	20	20	19	18	17	16	15	14
d20 base attack bonus	0	0	1	2	3	4	5	6 ,1
OS monster HD	8 to 9	10 to 11	12 to 13	14 to 15	16 to 17	18 to 19	20 to 21	22 plus
AD&D THACO	13	12	11	10	9	8	7	6
d20 base attack bonus	7 , 2	8 , 3	9 , 4	10 , 5	11 , 6 , 1	12 , 7 , 2	13 , 8 , 3	14 , 9 , 4

SAVING THROWS are not mentioned in stat blocks either book. Go figure. I guess we are just supposed to say that a monster saves as such and such class of such and such level. The grell is a weird, magicky sort of monster, with 5 HD, So lets say he saves as a 5th level wizard and call it good.

So yeah. d20 sucks. But it beats these murky old school rules. That is why there is an entire online community devoted to making old school D&D playable.