I'm considering rooting my (Verizon) Galaxy S4, currently running Lollipop as pushed by Verizon in August 2015 (plus any updates that have pushed out since then). There appears to be no likelihood that Verizon will release an update that lets me use an FM radio app such as NextRadio on this (three year old model) phone, despite the needed hardware being present (and Verizon now allowing FM radio on newer Samsung models, S6 and S7).
If I were to root this phone, either with Lollipop or Marshmallow (providing I can find a ROM for the newer Android version), would I be able to continue using it on Verizon's network, as well as use FM radio? If so, what do I need to watch out for? I can't (at this time) afford to replace the phone with a newer one, so it's critical that it continue to work as a phone with the existing SIM card, regardless what I do to it.