upgrading WordPress, or how to turn 30 minutes into several hours

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.