The initial goal and converting APFS to HFS+
I wanted to use Time machine for Mac OS backups for the first time. Time Machine backup does not currently work with a partition formatted with the new APFS format from Apple. The external drive and partition I wanted to use for Time Machine backups had a partition formatted as APFS because I had used it to boot Mac OS. The solution was to convert it to HFS+ (also known as Mac OS Extended). This is a destructive conversion but I no longer needed the drive for booting Mac OS on an external drive.
I used the instructions here for converting the partition to HFS+. This basically involves "erasing" the partition (and selecting HFS+ or Mac OS Extended) with Disk Utility in Mac OS. After the conversion, Time Machine worked fine for backing up to that partition on my external hard drive.
Unfortunate side effect
The unfortunate side effect was that the exFat partition I had on the external drive could no longer be seen by Windows for reading/writing.
The behavior was strange. Disk Manager in Windows could see the drive and the partition but couldn't do anything with that partition (and no drive letter was assigned). When I right clicked that partition (in disk manager), and selected "Change drive letter" or "Properties", nothing happened.
What went wrong and the solution
Apparently, somewhere along the way of converting the APFS partition to HFS+, the exFAT partition became inactive. The easy fix was to make the exFat partition active. I used MiniTool Partition Wizard (free version) in Windows to mark it active. Then ejected the drive, unplugged it and plugged it back in, and it suddenly was seen by Windows (10).
See other posts on Mac OS:
The external drive I was using is a Lacie Rugged Thunderbolt USB-C 2TB 130MB/S from Amazon