Can I upgrade one phone on my family plan at a time?

My wife and I both have iPhone 5's that are eligible for upgrades, but we want to upgrade one now and the other one in a few months. Is this possible? I had a bad T-mobile experience where one phone was upgraded and it kicked in as a new contract and they refused to let us upgrade the rest of our phones without paying full price.

Ironically, right after finding this out is when we decided to break contract and switched to the iPhone and AT&T.

Thanks for any insights!