Kalvan is partially right.
The L-frame has thicker dimensions (more "beef") through the pressure bearing areas of the gun.
The grips, however, ARE interchangable.
The K-frames do NOT "self destruct," though.
As with any gun that shoots high-powered ammunition, the gun will wear. It's the RATE of this wear (end shake and cylinder wobble are the greatest indications of wear) that has a direct relationship to the inherent structural integrity of the whole.
Being of lighter construction, the K-frames will show this wear before the more heavily constructed L frames.
A good analogy is riding a mountain bike vs. a lightweight racing cycle over bumpy trails.
The mountain bike will hold up for a much longer time, because it's designed to deal with the harsher, more demanding, trail surface.
This is true of the K- vs. L-frames. The K-frame was originally designed to handle the .38 Spl. cartridge, and was only late in life wedded to the .357 Mag.
The L-frame was custom built with the .357 Mag. in mind.