@OP: I completely understand where you are coming from. Since they are replacing your DU anyways it would be marginally more expensive to put in a P DU and you want ONLY that marginal extra cost forwarded to you. This is why they won't do it: Some people would become opportunistic and buy an S instead of a P and then try to destroy their DU to get a warranty swap and just pay the $500(or whatever TM internal cost may be) for the upgrade. So that person would now have a P85 for 82k instead of the 94k that everybody else paid, while TM would have an intentionally damaged S DU to send to the factory for repair. This is the same reason that TM makes an effort to give you a degraded HV battery if yours goes bad. Mine was down to 256 rated. It just broke and I got a warranty swap that charges to 255 miles). If they gave a 265 mile battery, what do you think people would do to their batteries after 7 years and 11 months?
For this reason, if they agree to upgrade you to a P DU it will probably cost you $20k as others have stated.
Since you think in money saving terms (otherwise you would just trade in your S for a new P, no?), figure this: Any used car that has been modified sells for less than a factory original spec car. Look at chip tuned cars and it is obvious. So even if you got your upgrade for free now, you would lose money in the future when it is time to sell the car.
If you need P, the most economical thing is to sell you S85 and buy a P85, new or used. And then you will get the metal pedals and the other details that make it "real" Just my 2 cents.