I think it's more a case-by-case basis. Thancred didn't really have a defined class/job, but rather a vague image that slowly transitioned over the course of ARR and cemented into what it is now with Heavensward. With the "Geomancer", it's not just the placeholder stuff, but the whole theme and everything brought up in that Astrologian questline is almost entirely Conjurer.
As for Arcanist, I would argue that 1.0 is probably almost as distinct from 2.x onward as any other FF game is from FFXIV, so that one to me doesn't really count.