If you renew it within 30 days (+/-) of expiration you will get the renewal discount of about $40 for a regular AP and about $50 for a PAP. However, the expiration date will be the anniversary of the current AP's expiration date.
Lets say it currently expires June 30, 2005. If you renew at the discount any time between June 1 and July 30, 2005 the expiration will be June 30, 2006. If you will not expect to be coming to WDW before some time in December it might be worth your while to just get a new AP at full price when you come in December, and then it will have an expiration date in December 2006.
I hope this makes sense.