Unfortunately not, once disks are assigned to an aggregate, the only way to get them back is to destroy the aggregate and build everything again. As you don't have any data in it, this is less of a problem, but not the solution you were hoping for.
Reinstall one head. Shut it down, reassign disks from other head, start installation procedure on the remaining ones. As you have no user data, could be an option.
Convert two RAID_DP into RAID4 which gives you 2 spare disks. Assign them to the head, create new aggregate and move root volume to it. Then destroy old root aggregate and redistribute disks as needed. Convert back to RAID_DP when finally done. May be easier as it does not require to redo all settings.
You have to take into account that when you create an aggregate, you are striping all of those disks as part of one large container. Each smaller part has been allocated to a part of the larger whole. If you pull out some of the smaller bits, you have to rebuild the whole. This is no different than a standalone server with a RAID controller.
With no user data, that is the way I would go. Configuring the filer again would not be enough to warrant my changing raid types, changing disks around, moving the root vol, changing the raid type back.