Your XJS looks great and you get many compliments every time you take it out. However, your level of pride lowers when someone looks at the interior i.e. a torn, worn or soiled armrest, console or seats, especially the driver side outer seat bolster. So your question is what choices do you have?
1) Replace just the side-bolster if it is the only area worn.
2) Replace just the center if this is the only area worn.
3) Replace just the upper seatback (very rare) if this is the only area worn.
4) Replace just the armrest if torn or worn.
5) Replace the seat covers.
I think you are getting the idea. You say well any one of these or all is the only problem but the rest of my interior looks great or you may use the word perfect!
Here is the question. are any of the choices listed above an acceptable solution? It is possible to find that one piece somewhere in the world that the interior has worn exactly like yours. For this to be the case, both cars would need to be in the same environment for all those years. However, the reality is this is an impossible task unless you are very very lucky.
Dyeing the interior to match your replaced areas
This is a good choice if done correctly. However, if any part of the seats is very shiny and seemingly very thin and or cracked, any treatment at this point is, in my opinion, a waste of time as all the work involved will most likely soon become a disappointment. Dyeing requires first a good cleaning to strip all soil and there will be plenty, then sanding, another cleaning and then two or more coats of the dye. And if you do not dye everything i.e. seats front and rear or both front if you only have front seats. The rear seats are questionable depending if they even show ware. Most times no one ever sat in the rear and they may be fine and you may be able to live with it. Door and rear seat or storage box area quarter panels may also need to be done. If you dye the seats you will need to also dye the console and center armrest.
What about replacing the leather and vinyl?
This is the correct and best solution for peace of mind and maintaining one's pride and not looking like a patch job. When taking this approach, the seat foam will be corrected where needed or replaced. The location of actual leather is any parts your body touches when sitting in the seat including the armrest on the doors and top of the console. The door panels, console and seat back panels are vinyl. I recommend asking top auto body shops for referrals and ask around and visit at least three trim shops and pay special attention to the surroundings like the people doing the work and if the shop smells of cigarettes look elsewhere because your car will end up smelling the same for a very long time because the smell will be trapped in the inner parts of the interior. Ask to look at completed jobs and ask them to show and explain what they will do and how. The right shop for quality work on interiors and convertible tops is much more important than price.