I don't know if they are going to do the "SF" (Short Frame) grip reduction thing with all the new models this time around or not. If so it will make the grip shorter front to back (less "hump" on backl), not thinner on the sides. I do like the 21 and 30SF 45's as compared to the regular models. I'm hoping the other calibers follow suit.
The mag issue is not related to the grip change at all, only the ambi mag release. New and old mags will both work in non-ambi mag release models. Only the new mags will work in new ambi release models. The 21 SF is available both with and without the ambi release, still waiting to hear on the 4th generation guns.