Do your car show the next gen seats in front & "old" type rear on the order confirmation (PDF)?
My P85D (currently in production) will be delivered with front & back next gen seats. I received confirmation by phone from Tesla European HQ. In the "Model S Order Agreement" document (the one containing the full price and without pictures), there is no separation between front & back seats. It just says "Black New Gen Seats". In the "View Design" pdf, the one that is generated on the fly, I get the picture with the old type seats at the back. This is because, on the website, there is no longer a picture available with the next gen seats in the back. You should not worry about that if you have ordered next gen seats all the way.
As others have mentioned, the folding down height difference is quite small. For me having seats with the same "horizontal design and bolstering" front and back is much more important.
I just got "the call" today for my Australian P85D order. The guy on the phone acknowledged that the difference in fold-flatness is minimal, and there was no mention of headrests. I was told that I could keep my order as is, switch to standard rear seats without refund, or upgrade to executive seats. I opted to stay with the next-gen.
Did you trade in your P85 to Tesla? I'm about ready to trade my S85 12/26/2012 and was pretty disappointed at the low value as the car has every option other than P.