I have seen many posts regarding troubles when upgrading Mac OS from Mojave
to later iterations such as Big Sur and Monterey.
For many, this advice is Monday-morning quarterbacking proverbially speaking …
One of the coolest things I (finally) discovered with Apple’s transition to APFS file system is the on-the-fly disk formatting capabilities.
When moving from High Sierra to later versions of Mac OS, I cleaned house and backed up the drive, and then formatted new Partitions, leaving the High Sierra volume alone, in which I could then clean install completely independent systems. One can also opt to simply add volumes (multiple containers) in which to install different Mac OS versions in order to test them out.
In choosing this method I think I have avoided some of the pitfalls of upgrading like problematic networking issues after a new system is installed over an existing one.
There are some cons to this in the need to reconfigure a lot of stuff on a new system, but to me seems preferable to having to troubleshoot anomalies arising from a new system upgrade flubbing up an already well-working one.
It’s definitely more work, of course if you need to keep Mac OS 10.12.Sierra, but even then it might be well worth the trouble to reformat a drive and re-install Sierra on a HFS+ partition, and add High Sierra or later on a separate partition in order to take advantage of APFS features