Please clarify your post? Is the model an 8330, 8830 or other variant etc? If it is CDMA (dosent have a SIM card) then there is a large possibility that other networks will not accept the device on their network. If it is GSM (more common and takes a SIM card) then all that needs to be done is for the device to be unlocked and another network's SIM card needs to be placed into the device.
Verizon and Sprint will not allow a device with another carrier's brand on their network. Flashing does nothing to change this as it all depends on whether the device ESN is in their database or not. Not their device, then highly likely the ESN is not listed.