There are cases, historically, where ships were made longer or shorter. I don't know about wider or narrower, but it's possible. Entire superstructures were changed in some cases, such as the Arizona going from her cage-masts to her tripod masts in 1929. There are also cases of armor being added or taken away.
Yes, the more intensive it is the longer and more expensive it should be, but it should also still be less expensive and time-consuming than making an entirely new ship in most cases.