Boardom, or what ever you want to call it, im not sure, but while I had a chance, I done a quick First and Last Xantia comparison...
The white Xantia is a XUd 1.9 TD SX model.. The blue car is a HDi 110bhp "Forte" model
This was because I happen to have the Sinker project and mums Y reg cars both on her drive at the same time...
Some external comparisons to start with..




Showing the nose mod, the rear lamp change, and the change from grey door moulds to colour coded.
The white car is a SX, mid spec on original cars, where the blue is a Last Forte, based on LX or even is equal to LX spec.
Rear end shape actually never changed through its life and they have the same profiles, but the front had the big change in 1998. Doors will swap across between cars, and the front end will swap across too, but we know this as we have seen some mixed models and ages on Xantias for sale on ebay before now.
Inside had more changes to the eye...
S1 Interior Dash...

S2 Interior Dash..

S1 Centre Console

S2 Centre Console

As the S2 had manual windows and no sunroof, thats the reason for he absent centre console switch's.
S1 Front Door cards, with NO Deep door pocket..

S2 Front Door pocket with the "handy" door pocket.

S2 door cards will fit S1 doors, BUT theres a slight position difference of the end Fixing screw behind the foldout lid of the deep pocket, so a small mod of a new hole is needed to fit them.. In fitting S1 door cards to S2 doors, the hole is still in the door for the plastic clip for the rear most fixing screw...
S1 Rear door card, featuring electric windows...

S2 Rear door card,

Rear door cards never changed at all through production, and will swap between cars..
The useual power house:

The newer prooven power house:


Note on the S2, the grill, now integrated into the bonnet opens with it..
Other difference's under the skin, were mostly at the front, the rear subframe and rear brakes Never changed, and were all the same on the TD and HDi, ABS was standard on SX early cars and VSX and an option on LX, by 1998 S2 it was standard on LX upwards also Fog Lights became standard on S2 cars. Both mirrors on the SX S1 are electric, on the S2 LX the passenger mirror only was electric. NOTE: S2 cars from SX upwards had both mirrors electric, and rear electric windows.
Air Con was an S1 car option to 1996/7 where it became standard across the range with no cost option sunroof instead of A/c.
The S2 cars had it as standard and no sunroff (except exclusive)
Anyway, thanks for reading, Hope I havent BOARD you too much...