Doesn't work like that, changing the firmware won't effect how the phone presents itself to the carrier's network or the features your device has enabled. Only effects the software on the phone and TMO firmware already has all the US carrier apps and will enable the appropriate ones when you enter the sim(via the MCM Client app iirc). Retail used to come without carrier bloat and more Gapps, but this is not the case anymore.
Just download the firmware and use a tool that interprets the firmware xml to produce the appropriate fastboot commands.
lolinet mirrors - powered by h5ai
mirrors.lolinet.com
lolinet mirrors - powered by h5ai
mirrors.lolinet.com
or
[Index] Flashing Motorola Firmware If your device has its own section please post in that section. You can ping me with @sd_shadow There are a few options for flashing stock Motorola firmware. Linux and Mac flashing options are posted in my...
forum.xda-developers.com
Only risk flashing for your same model. ATT's variant is xt2215-2 and could cause missing features or even brick if you try flashing it. Crossflashing breaks firmware updates so you have to download the full update and flash with servicefile.xml manually.