There are many problems that could occur.
A few right off,
Phones, when connecting to the network send a unique ID so the carrier will know what device it is.
While it may not be caught rite away it will eventually, sprint, will flag it because a Rumor wouldn't be downloading angry birds from the market. (example)
The legal issues, it's a federal crime to change the ESN off a cellphone with out the consent of the carrier, manufacturer and FCC.
In most states it's illegal to change any manufacturer assigned identifying marks or numbers. (keep in mind MEID's are assigned by the manufacturer and recorded by the FCC and then the carrier)
I don't have exact numbers or case law, but by federal statute it's a 3rd degree felony to clone a phone and can carry a punishment of 5 yrs in prison.
Other legal issues,
Telecom fraud (AKA phone fraud)
breach of contract
TOS violations
many others
Ultimately I say it's upto you, if the phone is worth even a 1% possibility of the legal troubles that might come?
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