Actually, the cylinder on modern J frames is long enough to accept .357 (it is the same length for both .38 Special and .357 guns) but S&W says the cylinder heat treatment is different for the .357. IMHO, if you want a .357, trade the .38 Special and buy what you want.
Before you do, though, consider that a lot of buyers of those guns in .357 either trade them off or load them with .38 Special. One poster said that in his area, gun shops are full of used lightweight J frames along with boxes of 47 rounds of .357 ammo. He says it takes just 3 rounds of that brutal recoil for the owner to trade down.
Jim