@Gloersen is correct that at least a large portion of the weight difference is down to the one piece deck and interior liner, and the new structural grid that would be required for the new deck tooling. In essence you end up with a double bottom thickness as well as the added weight from the gunnel sides and floor. While a 400lb increase seems high for a boat this size, and I would not be surprised if they are just more accurately stating the actual weight of the boat, the weight increase from the added fiberglass would not be insignificant.
Weight and balance are the two most important components in naval architecture, but are often the first to be ignored by a builder, so for everyone's sake I hope they were paying attention to it when they were updating the boat.