I have had two oem W7 to W10 fail on me, one of them was about a year ago, 120gb os drive waz near full, but the PC passed all the validation and qualified for the free upgrade.
The upgrade failed, the W7 was then reverted back and invalidated. A year later it never got sorted, I gave up basically.
My daughter failed last month, another oem licence. But i sorted a new key with that.
I have two PC's left with W7 ultimate retail keys, but do not trust they will upgrade.
One tbing, not sure if relevant, all my W7 were either dvd retail or oem, none had keys assigned to a microsoft account, but all registered to me.
My sons and daughters were re-done with their Microsoft accounts though failed.