Downgrade android 4.4.2 help?

So got a sprint galaxy s4. Took it to tmobile and the guy there was unable to get it unlocked, even though I have the MSL code. Said it was because of the droid version being 4.4.2, that I would have to downgrade it to 4.3 then it will work.

Problem. How the fuck do you downgrade? I have a 4.4.2 with the baseband NG2. All the things i seen about DGing a L720 are with MDC or MDL basebands. Nothing on the NG2.

Anyone got a clue?