I am not sure have anyone found a way to fixed the problem when you upgrade to WP 2.0 and have a problem using K2 theme but i have found a way so i am sharing with you all!
When you upgrade you will see
WordPress database error: [Unknown column 'user_level' in 'where clause']
SELECT COUNT(*) FROM wp_users WHERE user_level > 1
This is because they have change user_level to user_status what you can do is just change user_level in theloop.php in K2 folder to user_status !