with cingular, you can change plans anytime, no 90 day requirement. Also, they dont renew the contract, it just carries on (they look at the original date). Ofcourse, if your contract is currently expired and you try for the new rate, they will put you under a 1 year contract.
i have been changing my plans with cingular about every 6-9 months (there is always a better plan every 6-9 months) and they were always more than happy to help me with that. Infact, you can change plans w/ Cingular every day if you want.
PS: Cingular has been one of the best wireless provider for me in the SF Bay Area so far (~4 years now w/ cingular). I know that AT&T sucks big time in the Bay Area. Verizon is the best overall though. Sprint...dont even start. I have friends and relatives on all these different carries in the SF Bay Area and Cingular is the best in here. I do get dropped calls once in a while (like once in a week atmost) and thats only when i am traveling through low signal areas.