Why do you want to run ISPConfig with Apache2 ? If you have problems with port 81, you can consider to take the steps as described here: http://www.howtoforge.com/apache_reverse_proxy_ispconfig
No, I don't want to change isconfig apache server, I want to change the apache server on the server for the users, ispconfig runs a comilated apache2 and on the server I have been running apache1 and now I need to change it to apache2
ISPConfig does not run a compiled apache2, it runs a compiled apache 1.3.x. If you want to update your main apache, install the pacahe 2 packages from your linux distribution.
