5
I wish I knew.
If these are things that should've been done the first time anyway, I didn't know about it... but the succesful upgrade time, I did do the following (different) things first (I don't know if it affected it):
I turned off (deactivated) the third-party modules... and uninstalled WFChannel, WFFaq, Soapbox, and Tiny Content (they were just on the test site, so I didn't lose anything.) I reverted to the default theme, and I replaced the "uploader.php" in root/class/ that WFChannel had earlier replaced with a fresh copy. Then I did the upgrade.
What anything had to do with anything, I don't know.
On the real site, the only third party module I have up and running is a counter, and I deactivated (but didn't uninstall) it, and also reverted the theme to default (just for kicks.) I didn't want to take any chances... That upgrade went without a hitch.
So, if you think any of that helped (rather than me being a dork the first go-round and the XOOPS forum channeling psychic help), let me know, and I'll be sure to keep that in mind for the next upgrade another day down the road.
If at first you don't succeeed, try try again.
Then post to the XOOPS forum and try try again.