Thanks, gonZo. Now off to make a complete Time Machine backup of my Mac.
...and seven months later, it's done
Been putting it off and putting it off, but today I said, fuck it, let me do this. Backed up everything three times, including a Carbon Copy Cloner copy on two external drives, made a backup USB installer, and let 'er rip. Still getting used to the slight change in Finder (who asked for those stupid colored "Tags"? how come I can't scroll across to the left to get back up one or more folders in the hierarchy as easily as I used to?) but El Cap's looking mighty fine. I think I like it.