I understand your nervousness, but it is really not an issue. You will install 0810 on your system drive as normal - this is the only point where you need to be worried - just make sure you don't select one of your media drives as the system drive, and don't format the media drives with the partitioning tool. (We all know that you can screw up one drive and rebuild it with RAID 5, so you're really even safe from this scenario - but that's no fun, so just be careful.)
Once 0810 comes up, the raid array will be detected, configured, and added to the device tree. (In my experience, the detection of RAID arrays is now infinitely more reliable than it was in 0710.) You might need to change the PK_User setting in the web admin to use the correct directory structure (I believe it still defaults to "public"), but there really isn't any way to lose data during the process, since LinuxMCE doesn't make any changes to the data and the Kubuntu installer will only make changes if you tell it to format the drives (which you should NOT tell it to do).