dc3, I truly wish you would desist in believing that every response I make on a thread in which we're both participating is to you.
The line you quote is directed at the OP, and if you read what they said and what you said as well I believe that to be obvious. It is predicated upon your observation that the product code for Windows 8.1 Home would be embedded in BIOS/UEFI on the machine in question.
The OP may wish to follow your advice and go after the shop that installed Pro, and were I them I would. However, they may not be quite so willing to be the "warrior" and prefer to go back to Windows 8.1 Home. I just wanted to make sure that they understood that this could be accomplished without ordering anything from the OEM with regard to installation media.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall