I searched this out for you and according to their website, you don't have to renew your contract. Here'e the site and the question posed in the FAQ:
http://www.alltel.com/personal/cs/faq.html
"Do I have to make a contract commitment when I change my rate plan?
Topic/Detail
Wireless Service Plans
Anytime Plan Changes
Rate Plans
Upgrading My Plan
Question
Do I have to make a contract commitment when I change my rate plan?
Answer
No, a renewed contract or contract extension is not required for switching any rate plan. However, certain business accounts may require an extension.
Hope this helps.