The issue with a renewal is this.
My pass expires December 8, 2015. When I renew it, no matter when i renew it, I can wait to activate it until July if I want (you can only activate it when you are at WDW). But the renewed pass will still expire December 8, 2016. So if you are going to wait a long time before you activate it, you might be better off letting it expire and purchasing a new pass when you go back. It'll depend on how many days you intend to use it between activation and expiration.