You shouldn't have a problem at all. If you guys are at the end of yoru contract, you can certainly either end the contract all together, or just 're-sign' as individual numbers and accounts. Only one would be a resign, and the other would be completely new. At least that's my assumption.
I currently have an AT&T family plan with my roommate. We qualify for a 2-year contract extension, but since he's moving away this year I'm going to switch it to two individual plans.
How will that work in conjunction with a contract extension? We're both going to upgrade our phones, so I want to get the best deal possible, but it's vital that we both keep our phone numbers.