I was wondering has anyone ever had a "plain jane" bass and had an exotic top put on the bass afterwards?
Don't really see why that would be difficult; just take the existing top down to a level surface, glue on more wood, and reshape the top. Probably better to just make a new body though.
Some people have done it before but it requires power tools. http://www.talkbass.com/forum/f57/anyone-ever-send-bass-body-through-planer-put-nicer-top-341613/
There's a Fodera in the classifieds with just such a mod... http://www.talkbass.com/forum/showthread.php?t=993514
To take a contoured body down so it was flat would make the body thin, you'd need to slap an 1" thick top on it to make up for the difference. Maybe difficult isn't the right word, but buying a new body is certainly a better option.