Hello, do you still need help with this? I just made a mistake in the upgrade and ended up having to re-import the messages, but I believe I got them.
After applying jUpgrade, I reinstalled uddeIM's latest version since pretty much none of the components migrated. I also had to reinstall CB 1.8. This gave me errors when I clicked around the site about uddelib15php or something. Not realizing it was talking about a CB plugin I didn't upgrade yet, I UNINSTALLED uddeIM then reinstalled it, removing my messages table. After figuring it out and upgrading the CB uddeIM parts, I went to my old jos_uddeim table and exported it in phpmyadmin (this is the 1.5 version, which jUpgrade leaves for you but you can grab it from a backup as well).
I did a search and replace on the SQL file for jos_ and changed it to j25_ . Then you have to go into your j25_uddeim table and delete any PM entries, or you'll get duplicate primary ID errors. Then you can import the changed SQL file and it will have your messages.
Slabbi has mentioned about problems of user IDs not matching up, I saw. I think this has to do with the upgrade method? From what I could tell, jUpgrade seemed to keep the userIDs the same, although so far I have only looked at a few accounts.
Hopefully this helps someone.
