As far as I know, there is nothing proprietary about the S&W's that would prevent you from installing an upper in 22LR, or purchasing one of the conversion units that will allow you to fire 22LR's in the S&W as it is. If you decide to purchase a complete upper, keep in mind that you get what you pay for, so stay away from some of the cheap imported uppers.
The only thing you can't do is purchase on of the dedicated 22LR rifles and later add a centerfire upper - they will only shoot 22LR's becaure they are designed to fire 22LR's only.