It seems there wasn´t any converting. After the upgrade had finished i saw only 4 tasks running on the ACP . They were done after some less minutes. The posts with attachments that i checked were not converted. Same for the gallery and topics with youtube clips.
I suggest you make a subdomain like testdrive.yoursite.com. Copy your live forum to this new folder, import the database to a test database and run testdrive.yoursite.com .On it´s ACP at Settings: General Configuration change Path to 'upload' directory and Upload URL to the pathes of the testdrive folder. The right path is given here IP.Board has detected your uploads path as: . Just copy and paste. If you have other apps like gallery, blog etc installed you need to change the pathes there too. On Server Environment change the Cookie domain and Cookie prefix . You also need to update the conf_global.php on the test folder to the new pathes and database data. Now upload the 4.x.x beta and run the upgrader.
You always should change the pathes on the beta / test forum that you want to upgrade to this test forum. Also change the Cookie domain and Cookie prefix . Then save the database with these changes and use this one before you start the upgrader. If don´t do this it affects your live site.