Offers are subject to change, but typically we have been offered a discount renewal offer that covers any/all the AP's.
If you happen to be a FL resident, they have a special promo right now. Somehow you have to get a code at Wendy's on Coca-Cola cups, but it gives you 3 extra months if you renew before March 29.
For some reason though, trying to renew via the website has always been a bit flaky for me. I've always had better luck renewing over the phone. (1-866-PASS-4FUN) They also send a letter. Again, I can't speak to current offer, but prior letter was very funny with a Minion theme.
Also, like WDW, if you renew at the renew rate, your expiration date stays the same regardless of when you renew. Universal typically allows a small grace period beyond your current expiration date (at the renewal price)- BUT- the new pass will still expire on the original date if you renew at the renewal price.
If you only go a few times a year, letting your current pass expire- THEN renewing at full price- COULD be your best option.
Then again, with the new park opening...I would expect some changes to be coming in the very near future. As it is, I think they changed some terms a little, like blackout dates and food discounts. (but I don't know specifics)