If the phone is legitimately unlocked, then it will remain so. If it was hacked to illegally unlock it, it will, at best, re-lock to the original carrier. At worst, it will be completely unable to activate.
Did you buy it from a legitimate, authorized source, or from a gray market reseller? If the latter, odds are it was hacked.
Call AppleCare and give them the serial number. They should be able to tell you if it's locked an to which carrier.
ONLY the carrier it is locked to can legally authorize unlocking it.