I'm not familiar with RAID0+1 personally but generally breaking the RAID is a really bad idea (ie you can't just go from RAID0 to RAID5, just not sure about 0+1).
So I'm assuming you'll need to create a new RAID and migrate the data with the problem of not being able to move the data from 1 RAID to another. I would probably look to get the backup device first so you can back up whatever data you have. Either eSATA or NAS I'm sure would work.
Another option you could try is Acronis. You'd still have to have a backup device to store the image but you could take the current image (RAID0) and, after creating the new RAID (0+1, 1, or 5), you can use Acronis to slap that image on there. This is assuming also that you aren't looking for a fresh install. Theoretically, this is the quickest method.
My advice would be to get the backup solution first and back up all data before you start to play around with RAID's.