Not necessarily. From what I have seen of loaners is they come in batches at the beginning of the model year. And then they’re for a year or so until the next model year comes in. Rinse repeat. So once '17s are gone, it will be a while before '18s are available.
Maybe different dealers have their own rotation schedule, but what I described above is what I’ve seen personally.
Oops, this is from October (8 months ago), I’ve changed dealer since. Some dealers, though, may discount more if they know you’ll be servicing at their dealer.