With At&t you can. They will let you change from contract to pay as you go. Also as long as it has a sim card it can be changed over to pay as you go. Also talk to the company that you are using to find out if it can be done. You can check your phone yourself.
It depends what your current contract says. Now a days, contracts are permanent until they expire. Chances are if you ask to change your contract to a pay as you go, you'll have some sort of fees. Or, you'll have to get a whole new phone and number.