Extreme Noob question.

I have what was a Verizon Samsung S4. After switching SIMS from my old LG Leon LTE, and some help from MetroPCS, it is now running on the MetroPCS (T-Mobile) network. The phone is currently at Android 5.0.1 (Lollipop).I would like to upgrade it to Marshmallow. My question: Should my questions here about custom ROMS be addressed to the Verizon Samsung S4 forum, since the stock rom has not been upgraded or customized, other than being rooted, or should my questions be directed to the T-Mobile Samsung S4 forum, since I'm on their network now, and an upgrade of my rom, still using their network, is what I wish to achieve. As the thread title denotes, this is an extreme Noob question, but I'd appreciate an answer. Thank you in advance.