You dont have a dedicated Hotspare anymore You have a distributed Hotspare. Based on the used capacity of the disks the system reserved a portion of space on ~18 disks. You have received the increased performance of 1/18.
My calculator returns ~17.3TB of usable space based on 18x1.8TB on double Redundancy an 20% Raid10DM and 80% Raid6. We perform the sizing before we sell a system to a customer.
The SC doesnt convert form Singe to Double Redundancy silency under the hood. From my point of view every Blocks is handled like a new block written by a Host which means without changing the StoragePolicy every blocks goes into RAID10DM and than convert to RAID6. We do it once for a cutomer with around 40% used and at the end of the conversion we was on ~80. After some days and running the dataprogression system was down to ~60/65%.
You already have stored around 14.x TB so i thing there isnt enough capacity left for the online conversion. You should ask Dell if they can provide the math and confirm if you can do it.
But again.. there is nothing wrong with RAID5 and one spare. If you have configured SCOS and DSM DataCollector right and it'll send you an e-mail when an disk failure occured and you contacted dell as fast as possible, and Support Assist is configured as well you should get the replacement in 4h. You have the 4h MC support contract or just a simple NBD?
Regards
Joerg