Jason wrote:

MS "support" gave up trying to help me install 1709 atop
1607. They could not tell me if the issue was caused by
hardware or software and recommended a "clean" install -
i.e., you get to install EVERYTHING again, a daunting
proposition.

As you should know by now, you should have backups of
Windows, using Macrium Reflect. That way there is no daunting
task.