Despite all the things Apple says about 38mm bands not working for the 42mm model, I can say that 38mm bands fit just fine on the 42mm watches mechanically. The Apple Store specialist was nice enough to let me try the modern buckle on my 42mm SS watch and it worked just fine, clicked into place. Yes, there was a slight gap on the top and bottom of my watch, so aesthetically, it wasn't ideal, but functionally, it worked fine.
So if you have your heart set on a 42mm watch and a modern buckle, it will still work functionally, just not be the most ideal aesthetically!
So if you have your heart set on a 42mm watch and a modern buckle, it will still work functionally, just not be the most ideal aesthetically!