Hi, I think I have found the right procedure for the upgrade (at least for me). I tried the procedure on the wiki, but something went wrong, so I tried some more things and that is what I found:
1. backup your database
2. reset your database (drop all tables)
3. edit wp-config as needed (changing the table prefix to '')
4. run install.php
5. restore your database backup on top of the existing database.
6. Now, look with something like phphmyadmin at your tables, or go at your old config file, mine had this:
# database tables' names (change them if you want to have multiple b2's in a single database)
$tableposts = 'b2posts';
$tableusers = 'b2users';
$tablesettings = 'b2settings';
$tablecategories = 'b2categories';
$tablecomments = 'b2comments';
You can see the b2 prefix, this is for the tables listed, but the other tables simply had no b2 prefix. So first delete the tables posts, users, settings, categories and comments, then rename the b2* ones to the simply names posts, users etc. To do that I used phpmyadmin.
7. run import-b2.php
I think this should be useful....