My question is, actually my OS is CentOS 5.8, if I make a backup of my virtualservers, and I do a new fresh clean installation of the OS like CentOS 6.2 and install Virtualmin again .... I will not have problems restoring backups.



Howdy -- our recommendation for performing a migration is available here:

We'd recommend setting up a second server, rather than formatting your current one... that will allow you to perform testing prior to your new server being live.

Otherwise, there's always a risk of unexpected problems.

However, restoring the backups should indeed work, and migrating from CentOs 5 to CentOS 6 is supported.