Phone carrier

If the iPhone is not locked to a carrier then yes, you will be able to do that. If you finance the iPhone then you will have to pay it off before the carrier will unlock it.

Keep in mind that unlocked iPhones are only available directly from Apple in the US.

Devices sold by a carrier will be locked to that carrier.

Devices sold at other retailers will lock to the first carrier used. Do NOT believe the sales dweeb at Best Buy when they tell you the phone is unlocked. They are either ignorant or lying.

Yes.