You mean asking for them to swap the tires and wheels from your old 2015 to the new 2018?
or
You want them to put a new set of 17 inch wheels and tires on your 2018?
I have seen private parties sell new take off sets on craigslist of the 17" cx-5 2017 and up wheels and tires asking $600-800. The dealer will not be interested in swapping out wheels and tires on their other new vehicles to equip yours.
Try a tire and wheel site like tirerack.com, and get a set of nice lightweight wheels in your choice, and a set of much better than stock tires.