Juggling Win10/11 User and Version

1. I bought a new Win10 Home PC already activated via an OEM product key and a local account.
2. I already had Win10 Pro Retail, activated and linked to my MS account, on another old PC.
3. I managed to move (not fresh install) and activate and link the old Win10 Pro Retail installation (on HD) to the new computer, deleting the new Win10 Home installation. (Linux now on the old computer.)
4. Updated Win10 Pro Retail to Win 11 Pro on new PC. Working fine for 3 months.
5. I'd now like to re-install Win11 (10?) Home to the new PC using it's original OEM product key and sell it.

And save the Win11 Pro Retail product key (linked to my MS account) for another day!

Would just changing the product key on the new (now Win11 Pro) PC back to its origional OEM product key (was Win10 Home) and fresh installing Win11 Home (10?) do the job? It sounds too easy! What would be the sequence? (Not linking to my account.)
I could suck it and see but am a bit nervous about invalidating both product keys.

Sorry if that's all a bit spaghetti-fied! Merry Christmas!

The simple answer is that you will not invalidate any license for Windows 10, Home or Pro, on either computer regardless of what you do. Digital Licenses are completely different than product keys.