You can't exactly like change EVERYTHING on the phone to pretend it was from AT&T like logos and stuff. I'm pretty sure that Sprint uses SIM cards, my old Sprint did. Take that SIM and replace it with AT&T's SIM card. Go online to http://att.com/ and log in to your wireless account. Change some settings to fit what you need such as text, internet, etc.
nope... just Sprint... sorry... btw: me and my husband both have the LG Rumor.... and it sucks.... it turns its self on and off... just as a warning...