Hello,
My solution, performed on vCenter 5.5 standalone. vCenter is a VM.
Disk system should not be changed to Paravirtual. (moast probably SCSI 0:0)
VC downtime (maintenance): 30 minutes to 1 hour.
1. Connect ton the ESXs host and shut down the VC VM.
2. Edit VM settings and remove (but not delete from disk) all disks which had to be converted to Paravitual.
3. Add the disks back and configure SCSI as 1:0, 1:1 and so on , for each disk.
4. Change the SCSI controller 1 to Vmware Paravirtual
5. Now click OK for applying the new settings.
6. RDP (connect) to VC VM, and open Disk Management.
7. Bring Online all disks.
8. Reboot the VC VM.
Hope this solution works for you as well.