The update will roll out based on the region the phone is from. With an unlocked phone the carrier will have no control over the updates. For example, I have an unlocked 930 based off of France. Whenever the update is available to the france region phones that's when I get the update. I'm on the TELUS network in Canada. They're usually pretty good with the with the windows phone updates from what I've experienced (started my WP journey from them with the optimus 7) but they wont have anything to do with the 930 since they never offered It. I also had a tmobile 710 which I had to get unlocked but since it was carrier branded I was under the mercy of the carrier for any updates.
Just to clarify, carrier branded phones that are unlocked later I believe are under the mercy of the originating carrier. Unlocked phones that are not from any specific carrier will get the updates as they are rolled out based on the region it is from.