Smith & Wesson has NEVER marketed ANY of its M&P models as "convertible" weapons from .40 to 9mm, and the reason is simple.

If true, it is an unfortunate price that the rest of us have to pay (not being able to buy a same-caliber replacement barrel without sending in the pistol), all because an apparently significant number of people fail or refuse to follow the simple instructions of the manufacturer to use only that ammo for which the arm is chambered.

From page 9 of the M&P owner's manual:

"Locate the cartridge designation marked on the firearm. This information indicates the correct ammunition that must be used in this firearm (FIGURE 1). . . . . Never use a cartridge not specifically designated for use in your firearm. The wrong type of ammunition may exceed the capability of your firearm and may damage or even rupture your firearm."