If you want to do this with no downtime, be SURE that you've got a good backup of the data beforehand.
You can actually accomplish this without too much hassle using the "replace member" feature/operation on a PERC 9 card. The feature is intended as a copy-back operation from a hotspare to a replacement drive, so that your configured hot spare drive "stays" as the hotspare when not in use. However, the operation can be started manually.
The "replace member" operation would essentially mirror data from one of the drives in your RAID6 to a hot spare (which you should replace with a new larger capacity drive first). Since the data is mirrored, you're not throwing the entire RAID set into a degraded state each step of the way.
So the steps would look like:
- Unassign the current hot-spare, and swap it for a larger drive
- Assign the larger drive as the new hot-spare (this might not be necessary)
- Perform a "replace member" on one of the in-use smaller drives, selecting the new larger drive as the replacement
- Monitor the operation for completion
- Swap the drive you "replaced" in step 3 with a new larger drive, and repeat 1-5 until you're finished
Details on the "replace member" operation are on page 36 of the User's Guide
From there, you can actually expand the VD without downtime via "Online Capacity Expansion". Page 30 of the same user guide ("Reconfiguration of Virtual Disks" section) even mentions this "replace member" scenario specifically as a reason that you would have excess capacity to expand to.
Once all this is done, just perform a storage rescan from vSphere, and you should be able to expand the datastore size (though I think the VMFS filesystem might still be split into two pieces technically).
This whole process might not be the BEST way to do things, but it's certainly a Dell-supported method to get where you're going without the need for major downtime. But again, be 100% sure you've got a GOOD backup beforehand just to be on the safe side.