In the scripts directory, you will also find a script named "update_postcount.php". This script can be used to update the number of posts for users to the right value. So if you want correct numbers to show up in the user profiles, just run this script once AFTER the conversion has been completed. To run it, you will have to copy it over to the main Phorum5 directory. After that you can either run it from the command line or by opening it from your webbrowser. See also the instructions in the script itself.