Can I change my Mac's complete upper and lower casing?

Yes, you can swap out the bottom case and upper case assemblies on a 2014 Retina. But it probably won't be cheap.

iFixit has replacement guides for both assemblies. The replacement parts are both listed, but marked as "Not for sale"; presumably, these product pages are placeholders for a future time when iFixit has inventory from recycled computers.

MacBook Pro 15インチ Retinaディスプレイ Mid 2014 底ケースの交換

MacBook Pro 15" Retina (Late 2013-Mid 2015) Lower Case

MacBook Pro 15" Retina Display Mid 2014 Upper Case Assembly Replacement

MacBook Pro 15" Retina (Late 2013-Mid 2014) Upper Case Assembly

In the meantime, the parts listings provide us with the Apple part numbers for the two assemblies (Bottom case: 923-0670; Upper Case: 661-8311). This gives us the ability to conduct a wider search for those part numbers, such as eBay/Amazon/other online retailers.

PinnacleMicro stocks the bottom case, and both new and refurbished top cases.

PowerbookMedic also has bottom and upper cases.

All the various upper cases I've located include keyboard, trackpad and battery assemblies.

Be careful; apparently, each generation of the 15" Retina MacBook Pro has a bottom case with a different part number. There may be key differences between them, such as types and placement of screws. Get the 923-0670 model, or it may not fit.

Look at the replacement guides and the various replacement cases. If your MBP works, you may decide that the effort and expense aren't worth it, as long as the computer is currently functional. Odds are, the resale prices of the components will drop as more of the parts filter into the resale market; that probably won't happen until the original warranties expire, which is starting now (July 2015). Since this model was discontinued in May 2015, and extended warranties can take the coverage out to May 2018, it may take a while before the original owners start parting out/reselling the components.