If you have an exchange certificate or voucher for a new AP it does not expire and may be exchanged at any time for an active AP at no additional charge.
However, if you have a certificate/voucher for a renewal AP, and it is more than a year from the expiration date of the AP on which it was based, you will only receive the value of the amount you paid for the certificate against the full price of a new AP. To get the full value it has to be used within a year of the date of the prior expiration, and the expiration of the new AP will be based on the expiration of the old AP, not the date activated.