T999LUVUBOH1 and Android 4.3 - Any way to root/unlock?

So my mother just handed me her phone, and asked me to unlock it for her. She was on T-Mobile and wants to switch carriers, but for some reason T-Mobile won't give her an unlock code to do so. She definitely owns the phone, she's had the thing for years and paid it off long ago. Warranty isn't a concern, data loss isn't a concern (already backed everything up), I just want to get this thing unlocked. I hear the usual method for doing so is to downgrade to 4.1 and use the service menu (4.3 lacks the NW LOCK NV DATA INITIALIZ option) but I want to make sure I'm doing it right. I'm quite capable of following directions, I just can't quite seem to find any since OH1 is relatively new. I found some guides based on older firmwares, but I'd like some advice from some people with more know-how before I end up flashing a ROM that displeases the almighty KNOX overlord and bricks the thing.

TL;DR:
Have SGH-T999L on 4.3 and T999LUVUBOH1. Warranty not an issue. Need to know the safest way to unlock, and also ideally root. Kinda afraid to follow older guides without the go-ahead from someone in the know, since OH1 is so new.