Holy cow but you've got some difficult issues. I know that when I developed my RAID 0 config, I spent a couple months trying to find answers to RAID questions. A few people helped, but mostly I performed everything thru trial and error. I didn't have LVM, because I didn't need it, but I was amazed at my ability to wipe my drives, put them back together again, REDEFINE MY PARTITIONS, and still end up with /dev/md0 having the data I started with. (Usually, I was trying to erase my install and start over again. Occasionally I would lose it all, but often, I'd repartition and then put the partitions back and then rebuild md0 and it'd be like I didn't do anything at all.)
I don't know what FC6 is, but if it's an upgrade to FC4 then you should be alright unless the Changelogs say that RAID partitions are not backwards compatible.
If I were you, tho, I'd be breaking out DVD+RW disks and copying 4 Gigs of /video files to someplace safe!