Convert to another carrier

[jryker]

Looking for info regardless of device but I am wanting to know the correct way to flash a device to another carrier. No special upgrades involved. Just the correct way to say convert an Alltel device to U.S. Cellular factory ROM.
I have used the search and don't seem to find clear instructions.

[Rudegar]

you don't need to change rom to pop in a sim from another carrier and use it for phone and data purposes (if it's not sim locked of cause otherwise you need to unlock it)

but if you require some special program from the rom for your device provided by a different carrier then upgrading is the same thing as upgrading to any rom made for your type of device