*edit. Yes it appears you have to replace the buffer tube if you want an adjustable stock. You shouldn't need anything other than a new milspec buffer tube. You may have to drill out the pin that holds the current stock in place. If you can't do it yourself, it should take a qualified gun smith maybe 20 minutes to replace the tube.

First link when I googled "remove M&P 15 ft stock." Scroll down to post #7.

Changing pinned stock on M&P Sport 15