It took maybe 30 minutes to upgrade from WordPress 2.2 to 2.6, including time to revisit my notes on how I’ve done it before and to scan the official install/upgrade notes. But then it took a couple of hours to figure out why categories were gone and how to fix it. Long story short, I had to manually build the wp_terms table rows based on the old wp_categories rows. The SQL given in a forum note, lost categories after 2.6 upgrade, did the trick. But what a waste of time.