I dutifully backed everything up, ran upgrade.php and got:
advanced-cache.php) [function.main]: failed to open stream: No such file or directory in /home/1971/domains/momathome.com/html/wp-settings.php on line 69
Dang. Dang. Dang.
I tried re-uploading the files. No go.
A search found this page which explains how to remove a bad symlink file to fix the problem. I followed those directions, but that didn’t work because I didn’t actually have the advanced-cache.php file to remove.
So back to Google and a search for “wp-cache upgrade advanced-cache.php no such file” found the answer at last, and it was so simple that I could scream at the hour I wasted deleting, re-uploading, and ssh’ing:
in wp-config.php, it looks like the plug in adds the line:
Delete that line, save, upgrade goes perfectly.
It will be easier next time.