It is possible to do this, but you'll have two choices in doing it.
If you wanted to make sure everything was done genuinely with the manufacturer's shell casing you can pay Nokia to do this for you - ask them in their Live Chat service and they will direct you to your nearest Nokia Care store, where they'd help you out. This way you'd keep your warranty, too.
Alternatively the more popular way of doing this is via getting your own cheaper version from Ebay, and more than likely this won't be a genuine shell so expect it to be a little more brittle or that after a few months of use you might get some damage if you dropped the phone - these are complaints I've seen before. Also, manually swapping it out will void any warranty you have.