Google Compute Engine Upgrade from Debian 9 Stretch to Debian 10 Buster
Background I had installed a Debian 9 Stretch GCE test single-user instance that I now wanted to upgrade to Buster. This is a simple f1-micro “free tier” instance with Apache2 and PHP7.3-FPM, Docker, and with no database other than sqlite3. Both Apache and PHP versions were the latest from the sury.org repositories. In searches, I found very little info on how to do the upgrade. Note that Google's Debian installs have some differences from standard Debian images.
Some preparatory very helpful tips that I found are that (1) GCE instance boot disks can be attached and reattached, (2) a new persistent disk can be made from a saved snapshot of an instance's boot disk and then attached to the instance (basically swapping out the original boot disk with the new one created from the snapshot, and (3) a debian.org article on how to upgrade from version 9 to 10.
With this info, I felt I had two upgrade choices: