I see there is a new version of OctoPi out: v0.17.0.
In reading this link:
It says that OctoPi has a built in upgrader. However, if I run it, it does not update the version of OctoPi. Is there a way to do an in place upgrade of OctoPi to the Buster-based version without having to start all over?
Thank you!
No, it doesn't. It says that OctoPrint will offer to update itself automatically 
As written in the blog post announcing the new image:
“How do I update?”
There is currently no way to update from earlier OctoPi versions to a new version of that image. It’s not strictly necessary though - a new image basically means that if you have to setup a new instance you’ll start at a newer version of everything bundled with the image. Anything included on the image however can also be kept up to date without having to reflash. OctoPrint will have prompted you to update itself and will continue to do so, and most of the image itself can be kept up to date like every other Raspbian image.
So no pressing need to reflash!
If you want to however, create a backup.
Thank you sir! I knew how to update Raspbian, but always used "upgrade" not "full-upgrade". That's a new one on me!
Question though: if I use the built in backup/restore functionality in the current version of OctoPi (I'm running 1.3.12 on Stretch), could I theoretically simply create a new SD card with the latest Buster on it and restore that backup to have it retain all the prior settings, functionality etc? I checked out this page:
http://docs.octoprint.org/en/maintenance/bundledplugins/backup.html
...and it seems straightforward, but I am admittedly a Linux and OctoPrint newbie.
Sir Gina 
Whoops! I am just used to saying that. I really had no idea whether foosel was a man or woman!
Thanks!