Strange issue when changing post type

I am changing one of my websites (wpontwerpers.nl), in the current version all profiles are stored as a normal post, under specific categories. In the new version I'm using a custom post type called 'Accounts'.

When I try to change the current post type from post to account I can't find the post under Posts anymore (which is good, ofcourse), but it won't show up under Accounts either.
I've tried this with the Custom Post Type Switcher (or something like that) and also tested with changing the post type manually in the database. I even changed the guid in the table, to make sure it matches a few accounts I entered manually. But still: not there in the list.

The funny thing is, the account that won't show up in the backend does show up in the frontend. So in the frontend it looks like everythings okay, but I am unable to find it in the backend.
Any idea what's going wrong here? I've got about 150 profiles to change from post to account and - needless to say - I'm not looking forward to adding all those by hand (had to add all users by hand, since it's not possible to import users from a normal WP installation into a WPMS install..).