I am trying to make a 2024 version of my paladin. I can pick Oath of Devotion because it is free. I can pick the Xanathar’s options and Oathebreaker because they are from 2014 content I own, but I cannot pick the 2014 version of things I own if they’re in 2024 content I don’t own. I can’t use the Oath of the Ancients.
They haven’t ever stopped allowing usage of “legacy” content before, though. You can still use the original Kobold (with negative strength) instead of the new one.