IIRC the ex-TekSavvy ones I had weren't locked... it may be as simple as a factory reset without being connected to the internet - then adjusting the settings so that it loads the generic firmware from grandstream and doesn't re-provision from TekSavvy automatically.
I have notes about the setup but not about unlocking - suggesting that I didn't do anything major.
Specific notes I have include:
Manual was available at: https://documentation.grandstream.com/knowledge-base/ht812-ht814-administration-guide/
(Lots more notes for setup for Voip.ms, local dial plan, etc. I can post more or PM me if needed.)