An adjustable gas block or pistol length gas system should reliably cycle subs or supers in the blackout. It's a pretty efficient little cartridge, and I believe with the right powders can achieve almost total burn in 8-9" of barrel. As far as swapping the barrel in your M&P, it can be done. But by the time you purchase the barrel and gas system you could be halfway to a new upper. Complete 300 blk uppers can be had for around $300 without the BCG if you know where to look. They're completely interchangeable on your platform. Just move your BCG from one to the other. Not sure what your budget is, but something else to think about if you want to keep both calibers.
I also think this is the way to go. You basically get two cartridges in a modular package that can be changed quickly.
If you do this; I HIGHLY recommend you mark your magazines or even use different color magazines for each cartridge.
Another advantage is that you can go top-tier with your lower components and not have to buy them twice. Stocks, grips, and especially nice trigger groups add to the cost of a rifle build. I know my best fire control group cost more than the complete lower it went into.
Another thing that may save some money in the end is a quality optic with a repeatable quick-disconnect mount. Of course you would need a DOPE chart.