how many parts are interchangeable from a 2003 s type 3.0 6 cylinder to 2008 s type 3.0 6 cylinder jaguar? % wise.

 

There are a great many changes in the parts depending on the VIN number. Too many to list.

So perhaps the best place to look would be the official on-line Jaguar S-type parts catalogue.
Just look up the part you are interested in and see when and if the part number changed.

Here is a link:

https://www.jaguarclassicparts.com/uk/s-type-parts