Here is a PHP script to "decrypt" all the encrypted pm in the database.
Before using it, YOU HAVE TO SAVE YOUR "pmtext" TABLE.
To do it, go to PHPMYADMIN
If the script doesn't work for you, you will be able to restore your table
Then open "decrypt_pm.php", and add information to connect to your server. Save the file. Upload it to your server. Launch it with your browser.
You can now upgrade to vB 4 or whatever.
This script is not supported