This is a MAJOR No-No.
Occasionally, .38 Special guns show up at manufactures that have been converted to fire .357 Magnum ammo.
Alarm bells sound, sirens blare, and red lights start to flash.
Magnum guns are heat treated to a much higher level than .38 Special guns.
Many people think that gun companies are "ripping them off" or just charging more money for Magnum-marked guns.
They seem to believe that a gun company like S&W uses the same frames and cylinders on both calibers, and just stamps one as a Special and another as a Magnum.
"After all", the line goes, "There's NO difference between a Model 64 Special and a 65 Magnum".
Putting a Magnum cylinder in a Special frame will result in a quickly ruined frame, and VERY probably a "KA-BOOM".
A Magnum converted Special revolver is a live hand grenade. The only difference is, the revolver often doesn't blow.......right away.
Bottom line: Doing a conversion like this is one of those ideas that SOUND good, but are in fact, virtually suicidal.
If anyone else is standing nearby when it does blow, it becomes literally criminal.
I personally know of a case in the Mid-West some years ago in which a man converted a Colt Old Model .38 Special Trooper to Magnum.
The Old Model Trooper is a stronger gun than the "K" frame S&W.
When it blew, he mangled his right hand (he WAS a watchmaker up to that moment), the shooter next to him lost an eye.
The converter had to find a new line of work, got sued for everything he ever had, AND had criminal negligence charges filed against him, since the county prosecutor was on the line and saw the incident.