Took the liberty of putting a 15-17 Mustang and the new 2018 side-by-side. Granted the lighting is a little different, but you can see that many parts are the same.
In fact, here's what I can tell that they changed:
- hood
- front bumper
- fog lights
- headlights
Beyond that, I don't see any other changed, cosmetically. Of course, there are the other changes that you can't see (suspension, power, trans, interior).