Given the visual evidence between the Mk I and Mk II Constitution Class it seems as if they basically gutted the ship all the way down to almost the bare spaceframe. It was hardly a cosmetic change.
Given the extensive nature of the refit, it would appear you have to do double the amount of work that a new build would have. Givven the fact that you first have to take apart the orginal ship.
There seemed to be very little left of the orignal ship, that was visible at least.