You mean, you're upgrading from a standard AP to a Premuim AP? If so, the upgrade has the expiration date of the original pass, even upgrading on the last day you pa full price for the upgrade, and it would expire he next day.
If you are taling about upgrading a park hopper/MYW pass to an AP, it must be done within 14 days of first use, even if it is a non-expiring pass and would be backdated to expire one year from the first use of the hopper/MYW.