If you change a class design does a ship of that class change to the new design when you overhaul it?
there is a bug, if u change desing, which u used to build any ships, every one of that model will instant change. if u dont want to use this bug u have to copy design and change copied ver. (then use refit to)
This is not a bug, this is a feature that allows you to change reality if you don't like it. If Steve wanted the "unbugged" behavior he could have locked the designs when a construction of a new ship of that type was started long ago ... and promptly faced a rebellion from his users
The procedure you describe is correct for creating follow-on designs - copy the design and edit it. Think about e.g. Arleigh Burke Block 1, Block2, ... designs - the Pentagon didn't throw away the Block 1 "blueprints" when they designed Block 2 - they cloned them.
John