Most phones usually get updates over Wifi. This means you don't necessarily have to be on the original carrier's network to get the updates.
If you really want the carrier updates, you can always grab the ROM from places like Sammobile and update manually via Odin.
EDIT: Sorry, just noticed you said HTC. Obviously Sammobile will be no help to you since it's Samsung specific. First part I believe still applies though about being able to get updates via WiFi.
EDIT2: Looks like HTC has the carrier firmware on their website:
http://www.htc.com/us/support/rom-downloads.html#att