How much data do you have on the unit at the moment? a RAID 1 array consists of two equal sized disks that have identical data written to them i.e. they are mirrored. If you have more data spread over the JBOD array than will fit on the maximum size disk the unit can use you will then need to cull some of the data to get it to fit on a RAID 1 array. You will need to backup all the data on the NAS, use an external USB drive and then check the data is readable before you reformat the NAS to RAID 1. Then restore the data.
If you have RAID 1 array and one drive fails all you have to do is replace the failed drive with a new one of the same size, or larger, and tell the unit to rebuild the array.