I have an 1989 model and yes there are internal differences as they went forward. A few of the differences are, the boom is lower in earlier models this results in more sail area but sailing with the bridge is not possible. The differences is as much as 6". The fuel tank in the earlier models is located behind the aft stateroom and the holding tank is under the swim platform. In later models those were interchanged which results in smaller fuel tanks and larger holding tanks. The interior changes were basically in the sliding doors behind the settee and the dining table was no longer a white surfaced trimmed in wood but a veneer wood surface. There was also a difference in the side non opening port lights. I have not noticed any differences in the swim platforms other then the hatches opening in a different directions. In all they are great sailing boats and provide a twist of speed and ease of handling. Yes, the aluminum holding tank is a must to change out as soon as possible if it is still there. The CNG cooking system is another problem that should be looked into. I just recently switched mine and consider it great improvement. As far as sail handling, the continuous line roller furling system is also a desirable upgradeable item, since furling the headsail part way with that system is almost impossible to maintain in heavy winds. In all, it is a great and stable vessel and I would hardly recommend that model.
Bob