Change carriers from sprint to AT&T

I'm eligible for an upgrade, as I have had my sprint iPhone 4S for two years now. I have been eligible for an upgrade since June (2 months ago). Can I switch carriers, or am I locked into another two year contract with sprint?

As long as you haven't renewed your contract (generally done when you upgrade, but you don't have to upgrade your phone to renew your contract), then you can switch with no problems. If you already signed up for another 2 years (or 1 or whatever), you can still switch carriers but they'll charge you a fee, usually a few hundred dollars.