It turned out to be fairly painless. The installation went well. It took about a 1/2 hour.
Then, I started checking which applications wouldn't work. My biggest fear is that some unsupported legacy applications won't run and then I'm SOL. For example, I rely on iWeb to maintain my website. Fortunately it still seems to run.
All of my expensive applications worked. A couple of utilities like CyberDuck had to be upgraded, and there were a couple that relied on some libraries (X11 and MacTex) that seem to have disappeared. I downloaded the library packages, and re-installed them. Everything seems to be working so far.
As a bonus, a couple of software packages that claimed to work with Yosemite, but never did, now work correctly with Sierra. Most notable is LibreOffice 6. Under Yosemite, LibreOffice 6 would appear to start up, but it wouldn't do anything. So I'd had to revert to version 5.
