I'm currently running a script that is supposed to go through all the posts and rebuild the search - that it go through everything and mark all the key words. I'm not sure how long this will take, but till it's dont the search function won't work or be very limited.
Hopefully this will fix the error of searching only the posts that have been made since we switched over. After it's done we should be able to search everything no problem I made a backup of everything just in case something goes wrong ^_^