From our experience with Windows 8 upgrades, you're right out of luck. If your Windows 8.1 becomes inactivated for any reason, you can only use an 8.1 key to activate it. The keys are (intentionally) different and incompatible.
No, it didn't read an 8.1 key from anywhere. It just remembered it was activated and (because the Store is secure) it was allowed to stay that way. Upgrades via DVD seem to always finish up deactivated and needing a new key.
Your choices are (a) buy/acquire a new 8.1 key or (b) roundtrip through reinstalling 8 and upgrading through the Store. You can talk to Microsoft or your OEM supplier, but I wouldn't expect much help.