Have: Single partition firewire HD w/ 10.5.8 (my StartUp Disk), 2 bootable USB drives (1st= 10.6.7, 2nd= 10.8.2). Want: upgrade firewire HD to 10.8.2
You risk loosing your data in any upgrade.  You need to backup your important data. If your data isn't backed up, you do not have your data.

I recall you need to upgrade to 10.6 so you can do an online install of 10.8.  With an upgrade, the installation manager will not erase your data.  I have not tried this.

Robert