Question About Updating Samsung Galaxy S4 from KitKat (4.4.4) to Marshmallow (5.0.1)

[Android Central Question]

I want to update my old Galaxy S4 (SGH-I337, originally AT&T, but is now carrier unlocked and using MetroPCS).

It is currently on Android version 4.4.4, but I was thinking about following these instructions to update it to 5.0.1:
android.gs/update-att-galaxy-s4-android-5-0-1-ucugoc3-lollipop-os/

I don't know anything about manually updating Android versions. So before I attempt this, I wanted to ask if it is safe to follow these instructions even though the phone is no longer on AT&T's network? If I do this, will the phone stay unlocked and continue to work with MetroPCS as it does now?

 

[C_Button]

OP here,

Apologies, I obviously meant Lollipop, not Marshmallow. Whoops!

 

[Rukbat]

Since it's no longer on AT&T, you won't get updates over the air. You'll have to find a Lollipop ROM and flash it yourself (if you're lucky, you'll find an update that someone captured.) Your best bet would be the XDA AT&T S4 forum.