Your best bet is to move to a new server or use a temporary server to work with as a staging platform. You will need to install the same OS that is installed on the source server on the new or temporary system. Then backup the source server and dump the file to the new or temp server.
At this point there are several options:
Update the platform server from Plesk 6 directly to Plesk 7.5 Reloaded skipping any other builds. Plesk 7.5 Reloaded is able to upgrade Plesk 6 and 7 systems.
Clean and install the preferred OS and Plesk 7.5 Reloaded on the original source server then use the Migration Manager feature in Plesk 7.5 Reloaded now on the original server to migrate the customers from the temporary Plesk 6 platform server back to the original system.
Or use a combination of these options to backup and restore data to change operating systems.
To upgrade your OS and Plesk version without a temp system follow these directions.
Backup the entire system using the Plesk Backup utilities and dump the file in a safe place (off the system) ... Wipe the system clean and install Red Hat 7.3 or 9 and reinstall Plesk 6. Restore the dump file on the server running Plesk 6 on the upgraded OS... Upgrade directly to Plesk 7.5 Reloaded.
Backup again and follow the same procedure for upgrading to a different OS. There is NO WAY AROUND this process unless you use a separate platform system as detailed above.
Note: Plesk Backups are only restorable to the same version of Plesk.
(Example: P6 can be restored on P6, P7 on P7, etc...)