I think you should be able to. As long as at&t knows it's on their network and it's been put on their network. I could be wrong though, not sure if you would have to get updates from TMo since it's their phone.
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Rooted Droid 3: Fiance' spilled coke on it,retired now.
Rooted Droid 4
Rooted Droid Razr
Rooted HTC Evo 3D 4G(Virgin Mobile) with unlocked bootloader!!
Rooted LG Optimus Slider(Virgin Mobile)