An adult premium annual pass renewal is $495.23. A child premium annual pass renewal is $420.68. An adult annual pass renewal is $378.08. A child's annual pass renewal is $321.63.
As far as I know, Disney may, or may not send you a letter to let you know that you pass has expired, or will expire. It's probably best to know when it will expire, and plan accordingly.
Also, I think there's a 30 day grace period. If you renew after your anniversary date, your pass is backdated to the original anniversary date. So if your pass expired today, June 25th, they'd allow you to renew until the 25th of July. So let's say you renew on July 10 - your anniversary date would still remain June 25th. Make sense?