First it depends.
At one time the were different, couldn't tell you specific years though. Now they have the same external dimensions ie they are all "magnum" sized but the .38s are headspaced shorter since the cases are shorter. This way you can't get a .357 properly into a .38 cylinder even though the cylinder itself is technically long enough enough.
So let's say you had a recent manufacture Model 60 and you had a recent manufacture cylinder in both .357 & .38. You should be able to switch those cylinders as long as the gun didn't start life being intended to shoot .38 only.
I'm not a gunsmith and you should never do anything like that because some guy on the internet said so.