Ok I've actually done this for windows server 2003. I converted a Raid-5 to no raid and then back to Raid-1. The key is to have backups.
As far as I know, this only works with hardware Raid. The hardware Raid produces one virtual hard drive out of the several hard drives that are in the Raid. This virtual drive should be accessible to live cd's and such just as if it were a single drive.
I used the Acronis live CD to crate the backup of the Raid-5. CloneZilla should also be able to do a backup and restore of a Raid (if it is hardware raid) as described in their FAQ.
Once you have the backup of your current drive (no raid), you can create whatever Raid configuration you would like in the hardware raid settings manager which is either in the bios or in a menu right after the bios (creating a raid will delete all the information on the drives that you are using for raid).
Once the raid is created, you can use CloneZilla or whatever backup tool you used to restore your disk image back to the Raid virtual drive.
If the boot settings are correct, and your raid hardware supports booting hard drives (which it probably does) then your set.
By the way there are many different Raid controllers so instructions may vary. This is just how I found I was able to do it with the hardware that I was working with.