I did the same with Gryzor and made sure that part of the "deal" was that they honoured the part Ex price given to me in April for my Scirocco, no matter when the GTD arrives (August, September etc.). They are taking the risk that Scirocco price won't drop too far below what they can make money on, but to be honest that's their problem not mine.
And yes, I certainly have this in writing, even to the point where the dealer informed me that the deposit amount I would have to pay on the GTD would drop as I would have made additional payments on the Rocco in the meantime so my collateral in the car keeps increasing the longer they take to deliver the GTD.
It really is to their benefit to deliver my GTD ASAP to realise as much profit as possible on the Rocco.