Software unlocks don't really exist/apply these days. What likely happens in those scenarios is that someone knows someone that has access to unlocking in carrier side of things and gets the unlock processes that way.Yes an update wouldn't affect that, but how the OP found a single individual to unlock his handset the 'proper' way against Apple activation servers is somewhat questionable.... most of the time these individuals perform a software unlock, which relocks with an update, and are never to be seen afterwards......
I'm always weary about that kind of scenario. Maybe its just me being OTT. But nevertheless that's why I always purchase the handset from Apple outright.