回复：回复：不可以，AC-21是递交I-485以后的事，如果递交I-485之前

Why not?
Yes, and you don't even have to wait 6 months. PD is yours now.
The rule is this: Whenever you have a visa petition approved under EB-1, EB-2, or EB-3, and for whatever reason you do not use that visa petition but file a new one instead you are entitled to the earlier priority date as long as the second petition is also under EB-1, EB-2, or EB-3. Remember, however, that it is the approval of the petition, not merely of the labor certification, that allows you to establish and keep the earlier priority date.