Hello,   Unfortunately, there's not a supported way to switch out the 3.5" enclosure with the 2.5" one. I'm not positive, but I think that enclosure may be permanently attached by rivets. At any rate, you wouldn't be able to make the change without a bit of fabrication work. A new backplane would be needed, as you had mentioned, but you should be able to keep your RAID controller.
Hi there,   Does anybody know if the 321-BDUX : 3.5" Chassis with up to 4 Hot Plug Hard Drives be exchanged for a 321-BDUW : 2.5" Chassis with up to 8 Hot Plug Hard Drives for the PowerEdge R340 servers. We have two of these servers operating and have purcahsed a spare one with a 4x 3.5" hardrive enclosure. We were just wanting to know if these enclosures are interchangeable. This may mean a new harddrive backplane and raid controller card to???   Thanks for any feedback Mat
Dell EMC PowerEdge R340 Hard Drive Enclosures