Upgrading successfully from 2.0.2 to 2.2 was not as straightforward as it should have been.
I followed all the instructions, disabling all the plugins and setting the theme back to the original default, but when I ran the wp-admin/upgrade.php script I got a lot of database errors. The blog worked, but the categories were missing and the admin pages which refer to them showed errors.
These seem to be due to database permissions issues. Googling for a solution I found a reference or two (e.g. this one), so I restored the old DB (what a good thing I did the backup!) and used phpMyAdmin to change the db-user permissions to include ALTER. This appeared to fix the problem because the upgrade script worked perfectly.
My main gripe is that despite the (tedious, manual) DB fix, most of my posts appeared to lose their category tags. Eventually, these appeared again, but I'm not sure exactly why.
The upgrading process is irritating. WordPress is such a fine piece of work overall, it seems such a shame that upgrading should have been be so clumsy and buggy.