The fluid picks up moisture and can get thicker. You might leave the filter, but they are not expensive.
The new sleeve, filter, and pan will go another 18 years.

 

I've done three Jaguar Trans Fluid changes, 2 ZF6 and one ZF8. If you do it "by the book" its not that hard and well worth the effort. Just make SURE you pull the fill plug BEFORE the drain plug or pan; I had one that had an ornery fill plug took me hours to remove. The following video and .PDF should help....

 

Thanks!