I believe I was offered a small discount last year to renew prior to expiration. My So Cal pass expired the end of April & since I wasn't planning to going again til after summer (most of June thru Aug are blocked off) we waited til Oct to renew.
I think some/most passes offer $20 discount to renew. BUT unless you are planning a trip within a few week, it is best to let your pass expire and purchase new, thus extending the experation of the new pass.