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The reason for this is that everything in wordpress that accesses info in the db does so from PHP using the $table_prefix variable, and when you first installed your blog, a couple of options are set in the database that include that value, which makes it not really so dynamic! ;)

After rooting through my db, I found 5 or 6 values that still had the old table prefix in – mostly in the `usermeta` table, but also one in the `options` table. To make sure I got them all, I ran these simple SQL statements in phpMyAdmin to update all the values:

UPDATE `wp2_usermeta` SET `meta_key` = REPLACE(`meta_key`, 'wp_', 'wp2_');

UPDATE `wp2_options` SET `option_name` = REPLACE(`option_name`, 'wp_', 'wp2_');

(note: please be sure to backup your database before you execute any SQL on it – if something goes wrong, there’s not a lot you can do to recover it without a backup!)

In those two examples, assume the original prefix was “wp” and the new one is “wp2″.

With that done, I can now log in to my newly upgraded blog again! :)