Since your are on MK2, no more Loki.
You won't lose unlock status.
Search for the all things at&t thread, it has lot of required info.
All you need for the ROM to work with T-Mobile would be the correct APN settings.
Many usersdon’t see any particular reason to change out the stock firmware … more
28 Aug 2014
By Tomek Kondrat
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?