This is the relevant passage, right?But you can tho
When playtesting the new version of a class, you can use a subclass from an older source, such as the 2014 Player’s Handbook or Tasha’s Cauldron of Everything. If the older subclass offers features at levels that are different from the subclass levels in the class, follow the older subclass’s level progression after the class lets you gain the subclass.
What they don't explain here is what happens when your subclass started at level 1 or level 2. Do you get your level 1-3 features all at once at level 3 in the One D&D class? Do you lose the level 1-2 features and only get stuff you would have gotten from 3 forward?
And if the old subclass level progression doesn't sync up with the One D&D subclass feature levels, do those become dead levels?
Did they explain this more thoroughly outside UA? Genuinely asking.