Data will be preserved if you're careful, but a complete backup is highly recommended.
If you have an Intel based system: http://communities.intel.com/thread/28670
For an AMD based system, this may help: http://forum.overclock3d.net/showthread.php?t=30211
You need to make sure that the RAID drivers are loaded at boot time before the migration. In other words, you should be able to boot from the single drive when the controller is in RAID mode. http://support.microsoft.com/kb/922976