I looked at my old notes for what needs to happen to update a 34.11 mod to a modern version. Among other things:
All the creatures need to be given gaits and [ATTACK_PREPARE_AND_RECOVER:3:3] tokens to replace the old SPEED system. The appropriate male creatures will need geldable flags in their castes, and you'll want to give certain creatures LOCAL_POPS_CONTROLLABLE and LOCAL_POPS_PRODUCE_HEROES for beastman adventurers and OUTSIDER_CONTROLLABLE for civilized races with outsiders.
INDIV_CONTROLLABLE can be removed from the entity files. It used to be how outsiders were generated.
Civs need the new values, instrument raws, and the various scholars. They also should have mercenaries.
The bodies need to be switched from HUMANOID and QUADRUPED to HUMANOID_NECK and QUADRUPED_NECK.
Smells and vision arcs might be nice, but aren't essential. On a similar note, you won't have demons or vaults if no civ has Dark Fortress sites. Like before, it's not strictly needed, but it's nice to have.
STRANGE_MOODS must be added to at least the playable civ or you won't have any artifacts or, well, strange moods.
ALL_MAIN_POPS_CONTROLLABLE should replace ADVENTURE_TIER.
SELECT_SYMBOL needs to be adjusted to deal with libraries.
You'll probably want to put NO_ARTIFACT_CLAIMS on any kobold-derived civs.
I know plants and trees will need to be reworked, but I've never really dealt with them.