AT&T has announced that it'll hand you up to $650 in credit should you choose to switch from another mobile carrier. If you're prepared to jump through the various hurdles, you'll be entitled to a pre-paid gift card equal to the value of your ETF or device balance. In addition, the network is letting you pair the deal with its buy one, get one free offer, enabling you to grab two shiny new devices at the same time. The offering is the latest in a long series of credit offers, with Verizon pushing its own offer to $650 last December.