How feasible is it to upgrade a phone on contract and sell the old one?

I've done this a few times with sprint.

just make sure that you clear everything off of your phone, and be very descriptive of and scratches or loose buttons the phone may have

 

I always do this with t-mobile (UK). By the end of your contract (after 18 months) the original phone will have lost a lot of its value, so replacing it makes a lot of sense. As long as the new phone is on the same network (or unlocked) there is no problem.

 

I do this all the time with my old phones. Just sold a Blackberry 9000 and 8900 on Craigslist when me and the woman got the Droids.