Originally Posted by
llkashll I'd assume that upgrading from Win 8.1 -> Win 10 would preserve any carrier locks, but assuming locks are enforced at the OS level, a revert from Win 10 -> Win 8.1 should wipe any carrier specific information (since the revert process uses a stock build and doesn't retain any configuration).
The WDRT process downloads an image that is
specific for your phone. So it comes with all the carrier bloatware, if any, and all. I have a 640 that was carrier locked on WP8.1, carrier locked when upgraded to W10M, still carrier locked when restored to WP8.1. I got the unlock code from T-Mobile and now it is unlocked with either WP8.1 or W10M. I think the carrier locking is maintained in the hardware, not in the OS/firmware.