I caught this 'undocumented' feature (re: changing of user id's) during a test upgrade on my site.
The fix I ended up doing was to comment out the importer code that deletes the user_id mapping table. This table has pre-update and post-update user_id's mapped out and made updating my Photopost db pretty simple via an UPDATE statement grabbing the new user_id via a subselect.
You should be able to re-create the mapping table by comparing login names from your old db and your new one.
