I cannot speak from direct experience with the release on the S-family, but from a DOT RAID perspective, you will need a spare disk in order to convert a RAID4 to RAID-DP.
If you are asking if you can fail a diskand build a "degraded" raid group into a RAID-DP vol, what would that gain? The system will then be running a RAID-DP degraded volume that can only sustain a single drive failure still.
The only procedure that sounds like it will be supported if you were at a disk limit would be to backup, destroy, and recreate the volumes on the raid groups you want to convert; a disruptive task.