I also did the debranding on my ATT Lumia 635. It no longer shows as AT&T, and I can no longer revert it back to 8.1. I'm stuck with it on Windows 10 Mobile. The tools WILL NOT revert it if you have debranded it, because the tool gets confused. It looks in multiple places to find what it is.
I'm wondering, though, if we go through the debranding procedure to make it think it is the ATT phone again if it will be successful, even if it doesn't have the ATT ROM. Maybe the registry entries will be enough? Worth a shot later, when I have time. Probably some time next week. If someone else gets a chance to try sooner, let me know here.
While I really don't mind it being on Windows 10 Mobile, I was thinking of giving it to my mother-in-law as a backup to her current 635, which is a branded ATT 635 GoPhone. However, there are too many crashes on W10M on this 512MB device to just give to her, and she'll want the same OS she has now so that she doesn't have to "relearn" anything. I do have another 635 that was not unbranded, so I'll try to revert that one.