Definitely time to dig into Dell's documentation on how that particular RAID set is managed and what its capabilities are. If it'll support bigger drives (either the ones that the customer wants or some other size) then maybe that solves your problems for you.

In other words, buy the bigger drives, pop out half of each RAID-1 pair, put the bigger drives in, let them synch, verify the new drives are good and have all the data, and then repeat the process for the second half.