New rule--we (and by "we" I mean everybody in the blogosphere, including you) are calling it Type ___ D&D from now on. As in, "Type 3.5 D&D" or "Type 4 D&D". This solves many philosophical problems, plus makes all the versions of the game sound like demons, which is good.
I can only live with a house full of Type 3.5 books so much longer. My rapidly levelling-up players have begun to creep around the Player's Handbook and ask unsettling questions like "What's a skill point"? I fear soon they will all want a level in Demonborne Fleshtoucher or some shit. Plus the original books look so much cooler lying around your house. I am soon to re-acquire the AD&D books.
Problem is, once you give someone a toy, you cannot take it away. So I have to stitch together a mongrel game of the old rules which I like, and the new rules which everybody here is used to.
So, Type Mongrel:
-You can be whatever class or race from whatever type D&D you want. Except, for obvious reasons, any kind of bard.
-Experience point awards and levelling-up will be as 3.5. No x.p. for traps.
-Beholders will appear to be spherical eye-ball monsters rather than the deformed product of a night of incautious passion between a bell pepper and the Kree Supreme Intelligence.
-To hit and AC will be as AD&D so I can use the chart.
-Hit points, damage, and ability-score bonuses will be as 3.5--except the base-attack bonus, which won't exist (melee and missile bonuses will).
-# of spells and class abilities will be as AD&D except what anybody's already got when we make the switch at the beginning of the game.
-Saves will be as 3.5 because come on AD&D. Wands, staves and rods?
-Thieves will have whatever skills (a "skill" was/is interpreted as the privilege of re-rolling a failed ability score check roll to do the relevant thing--that is if you have "climb" you can roll twice on your strength to climb a rope and use the best result) they had from 3.5 before the switch + normal AD&D pick locks, find traps, etc. chances. However, no new skills may be acquired after that unless you can make a really good case for it or are exceptionally attractive.
-No feats, skills are only for thieves.
-Barbarians can still Rage as in 3.5.
-Spells you already have remain as-is. As for new spells--if you can find it (in any type of D&D or any other game, for that matter) you can have it. Precise effects of non-AD&D spells may vary slightly from what's printed, according to DM fiat. A standard sleep spell will generally get a save. Anyone using anything from Eldritch Weirdness will be considered totally badass.
-You can use any weapon or armor you've already got, but unless you're a fighter you can't just pick up any old new weapon or armor once the switch happens and expect to be good at using-/moving in- it.
-Racial bonuses will be 3.5. Gnomes will be treated as dwarves, only midwestern. Dark elves will not get a level-modifier for purposes of calculating x.p. because I forgot and accidentally levelled Frankie up early and, hey, considering how new she is to all this and how she almost ket the party get TPK'd twice due to having forgotten her Drow's once-per-day abilities, I think it's ok to handicap her a bit.
-All monsters and other foes will be custom-built to fit Type Mongrel stats.
-The top halves of Mariliths will look like hot chicks--as in AD&D, but will appear to be capable of ordinary movement and locomotion, as in Type 3.5.
-If you want to take a level in something, as in 3.5, I say ok.
-If you want to try to kill a god rather than some Mickey-Mouse "aspect" of the god, be my guest, just remember its HP and damage will be 3.5 scaled.
If anybody reading this sees any major translation problem I haven't foreseen, please speak now before it blows up in my face.