It would be a quick change I agree but it isn't unprecedented. The Unibody 15" MBP launched in late 2008, it featured a removable battery. The following year, that was gone. I admit this isn't as drastic as changing the entire enclosure design but it shows they aren't afraid to alter a new design on a second revision product.
Another thing to consider is that the Retina MacBook Pro design is very similar to the Unibody MacBook Pro. It's thinner, it has those air in-takes on the sides but it still looks mostly the same.
It kinda reminds me of the Intel switch. They went from the PowerBook G4 to the MacBook Pro whilst using the same basic design. The MBP had an infrared port and the built in iSight but apart from that it was basically identical in outward appearance. Then they introduced the Unibodies about 2 years later.
Maybe they intend to do that again?
Personally I think I'd like a black one aslong as the finish is to a high quality and lasts a long time.