Precocious Apprentice and Retraining.

[Yogibear41]

If at level 1 I take Precocious Apprentice on a Wizard granting myself a single 2nd level spell, which I then use to qualify for and take as a 2nd level 1 feat a particular Reserve Feat keyed to it, upon reaching level 3 and learning to cast 2nd level spells normally could I then Retrain my Precocious Apprentice feat to another feat legally while keeping the benefits of the Reserve feat I took at level 1 since I can now normally cast 2nd level spells?

[Zanos]

I believe you have to retain a legal combination. So you can't take the reserve feat at first level if your current feat arrangement wouldn't have allowed you to cast 2nd level spells at first level, since you wouldn't qualify for the feat when you took it. You could retrain the reserve feat to something else and then use your 3rd level feat slot to take it again.

It's debateable this combination works to begin with. Precocious Apprentice has special language for "When you become able to cast 2nd level spells", implying that the ability it grants does not allow you to do so.