doctorbadwolf
Heretic of The Seventh Circle
Well yeah, it’s like reprints and errata.Whatever the rules the DM says apply to you, apply to you. I'm not sure what kind of shenanigans you're expecting to get away with here lol.
My expectation is that most tables will do it like this:
1) General rules - all from 1D&D (grappling, skill DCs, etc. etc.).
2) Class/subclass-specific rules - all from either 5E or 1D&D as appropriate to the version of the class. As a DM I would not personally let people "cross the streams" and take a 1D&D subclass with a 5E class or vice-versa, or not without checking it carefully.
3) Spells - Depends on the DM but I expect most DMs will just use "all from 1D&D" except where spells aren't present in 1D&D.
You use the most current version of anything that’s been updated or reprinted.

But I see no issues so far with existing subclasses and new classes.