On the sixth (!)* attempt, I managed to hammer in Windows 10 on my test machine. Well, sort of, as the install refused to use the existing video driver(s) from win7/64 already present so the screen is garbled, neither wi-fi nor sound works, and I lost my printer.
* each attempt downloads 3 GB of install media, over and over again, although the media files already are present on the PC. A hallmark of "artificial intelligence" I presume.
Oh, and how could I forget to mention the machine now is glacial slow?