Can I replace a phone without a new contract ?

I'm considering to get a new phone with a certain feature which my current phone doesn't have. I can get the phone at low price from somewhere, but I wonder how the contract goes if I replace the phone.

I'm already out of 2 year contract. If I get a new phone, say, from ebay, and use it with Tmo, can I keep the current calling plan without 2 year contract ?

What would be the cost incurred just to switch a phone other than the cost of the phone ?