I know we can do that, but it's a cumbersome method, and unless you change the new design name to match the previous class you get 2 different update path.
Example:
You have a BB of the Bismark class, you make a 1941 refit, copy that refit to a new design to build it. Now if you don't name that design "Bismark" you have 2 diffent class of ships that must be refitted separately, exacerbating the problem.
So the question remain: why we can't just build refit and get over with?