I used Webmin for it
First I installed using the guide: Securing Your Server With A Host-based Intrusion Detection System
After that I switched to Webmin and went to system --> users & groups. Here you can click on the group and change the ID. I just made of the first 1 a 2 to keep it simple
. After that you click on each ossec user (4 total) and change the user id and add the new group ossec as primairy group (ignore the postfix errors).
Than I went to /var/ossec and change witch the File Manager in Webmin all the files the user and or group. Only when it wasn't ossec allready but 10050 I changed it to 20050 for example (click on the file and then click the info button of the file manager). You have to do this with all the folders, subfolders and files. (don't forget the id of the ossec folder itself)
After you have changed all the files you can start OSSEC by command or by Webmin (system --> startup & shutdown --> ossec --> start now).
It started perfectly in my case. And after a few seconds I received the first ossec email that ossec started