I've run several 3.5 modules in 3.0 with no modifications, so maybe conversions aren't needed? +14 attack bonus is still +14 attack bonus, even though it might be +-2 from the 'correct' value. It's not like the players would easily know if they're fighting 3.0 or 3.5e version of the monster.
Well, the earlier modules can be run more or less as written, but once you get to the later ones... They start to be full of spellcasters, and many of the spells have been changed. Especially Haste. The "tactics" section for every spellcaster starts "In the first round, he casts Haste and xxxxx". Obviously, that will need to be rearranged.