Hi, i migrated my old forum to the actual ipb version. As it runs several years, with several addons on several board-software (ipb, phhpbb, joomla, ipb.... and so on) i want to clean up my database so it has a fresh start but keepin all my users and threads.

Somebody can advise me?

Can you create a localhost or demo installation? You could simply do a new installation of IPS, and compare. 

All of your IPS tables should have the same prefix, as well.