The RAID array will be automatically re-detected and reinitialized after LinuxMCE is installed. There will be no need to rebuild the parity drive, and no data loss. This feature existed in 0710 also, but it is much more reliable in 0810. Worst case scenario - LinuxMCE does not re-detect it, and you have to manually reinitialize it using the mdadm tool. This is quite simple. You are not at risk for data loss - don't worry.
Assuming you're ok with MySQL, you can get all that data imported properly without too much effort. Just dump the important tables from the mythconverg DB (you cal look on the MythTV site to figure out which you need - I can't remember off the top of my head) from 0710, and re-import it AFTER the Kubuntu install but BEFORE you start the LinuxMCE installer. When the MythTV packages are installed, the database will be upgraded to the new schema. You may have to do some global replacement if your file paths or tuner numbers change.
Also - look into the MythTV version issues floating around, and apply the fix if it's not already there. http://svn.linuxmce.org/trac.cgi/ticket/574