The firmware is the same, and so is the modem. What differs between the two is the CSC, carrier specific content. You'd have to check if Fast Dormancy and HD Voice still work, but those are about the only two tweaks that changing CSC might affect.
I don't know for sure about fast dormancy, but HD Voice required a factory reset to activate in some circumstances.
There is a code to change the CSC from another already available on the phone, but I don't remember it off the top of my head (it's been posted on here a few times already). Caution, it does wipe the phone without warning.