@ shadowcreeper_1: It depends. Did you buy the device *from* ATT, or TMO. If ATT, then no, because the boot loader is supposedly perma-locked, which means no custom recoveries, ever. You may have been able to root, but that's about as much you can hope to accomplish within a reasonable time frame.
If you bought from T-Mo, it should work as long as you never took the MM OTA update (or manually flashed it). My method of returning the phone to the store for exchange might work (to get a same device running Lollipop) if you had the same predicament as me. But you have to hurry because soon the devices T-Mo sells in store will have MM update already factory-installed.
Another approach is to buy a phone you want from T-Mo, unlock the SIM card, cancel the contract, then go buy service from ATT. This will enable you to have service from ATT while still having having the T-Mo version of the device that is fully hackable with custom recoveries/ROMs. Downside is you'll take a credit hit if you cancel the contract while the device still isn't paid off, and you risk the device's ESN being blacklisted after switching to ATT, which means no cell company in the US can sell you service for the phone, essentially leaving you with a wifi-only device. So it's better to just buy the phone outright off-contract/prepaid, paying the full price upfront, then get the SIM unlocked and switch.