I was afraid that I would get the new price as well as my SA. He chatted with his manager, and his manager said the price wouldn't be the new price. Then, he submitted a ticket on his laptop, I went home and saw new update with Y Performance, and the price is $63,990 which is the old price.
So in my opinion, they don't go by the current price, they go by the price at the time you order, then add/subtract the difference when you upgrade/downgrade. For example, MYLR with 20inch wheel and MYP have $3,000 different , so they just add $3,000 to upgrade.
That is what I think. I still got the old price by the way.