Re: HTC one m8 for windows
One M8 was only offered carrier branded with Windows Phone, so far as I know.
In that case Updates come from the original carrier, no matter if the SIM tray is unlocked or regardless of where you use it around the world.
Now the good news. One M8 is able to recover with Windows Device Recovery Tool. So if anything goes wrong you can reflash 8.1.
Get the phone and get on 10 Insider. Use the slow ring for stability. Bump up to 10586.29. Un-enroll from Insider and Hard Reset. Now Microsoft will send your OTA updates.
Feel like a BOSS.