Renewal rates are less than a new purchase. It’s not a lot, but every bit helps.
Usually, the renewal rate is valid for up to 60 days prior to the expiration date. You can’t renew any earlier than that. Your old pass remains valid for use until it expires, and then the new renewed pass becomes valid, with the same expiration date a year later.
The renewal rate is also valid for 30 days after your pass expires. If you renew after expiration, your renewed pass is valid for one year from the original expiration date, not from the renewal date. If you wait more than 30 days, you are buying a new pass, if they are available.
Of course, Disney can change this at any time.