An update on the upgrade ...
First, thanks to everyone for your advice. @kris455, @BrianK, @OldNavyGuy, @TeraByteSupport -- what I learned from you definitely made the upgrade run smoother.
The hardware upgrade went exceptionally smoothly, and the new system is amazingly fast. All of the drivers were found by Windows and the Gigabyte Control Center automatically. I used IFW multiple times during the process to make backups to an external USB drive at various steps, and once to copy/clone the old hard drive to the new SSD. IFW was extremely easy and straightforward, and did everything I'd hoped it would do.
IFW was definitely the right tool for the job!
The only problem I have is licensing. Although
slmgr /dli says I have the retail version, I looked through my records and found it is actually the OEM version. Because I don't have a Microsoft account (and do not wish to get one, but that's another story), I could not contact support. I spent many hours searching over the last couple of days to see if I could upgrade.
This site says: "The November Update now describes all licenses upgraded, whether full package or OEM, as retail. Even though your free upgrade to Windows 10 from an OEM Windows 7 or Windows 8/8.1 is now identified as retail, in spirit, it is still an OEM license."
This 4-year old YouTube video shows an upgrade and activation of Windows 10 that was upgraded from Windows 8.1 OEM, and it was reactivated with the original Windows 8.1 license; however, a comment from 9 months ago says: "After September 29, 2023 Microsoft no longer honors windows 7 or windows 8 keys unfortunately.". This is close to the time of the change described in the previous paragraph.
It seems I'm about a year too late to do a transfer with the original license. I only reasonable conclusion is that I have to buy a new Windows license to be legal, which I'll do tomorrow.
On the positive side, I suppose I could recreate my old machine in another case and use it as a spare Windows box as long as Windows 10 is still supported, and after that convert it to XCP-NG server running Linux boxes. But I probably won't bother.