There is an ATF letter around somewhere that somewhat clarifies the rifle and pistol conversion process...although I can't find it right off hand.
What I'm going to give examples of is with an AR style platform since it's easy to change because of the upper/lower receiver. It would work the same with rebarreling and action of a firearm where the upper doesn't quickly detach and reattach.
If a firearm starts it's life as an pistol, it can then have a longer barrel attached to it and it's still a pistol. If the barrel is long enough and then you add a shoulder stock to it and meets the barrel and overall length requirements, it becomes a rifle. After this, the stock can be removed and it's a pistol again. After this step the barrel could be changed to be shorter than required for a rifle, as it is again a pistol. If the barrel and overall length are too short before the shoulder stock comes off, it's a short barreled rifle and subject to NFA regulations ($200 making/transfer tax, etc. - stiff fines/penalties for illegal posession)
If a firearm starts it's life as a rifle it can't be converted to a pistol. It can be converted to a short barreled rifle (NFA, $200, etc.)
If a firearm starts it's life as an "other" (not an NFA item Any Other Weapon or AOW), it can be built into either a rifle or pistol. This is usually the stripped lower. If it's built into pistol first it can be converted back and forth per the example above. If it is built into a rifle first it is a rifle unless you go through the NFA process to make it a short barreled rifle. Because an "other" can be built into a pistol, an FFL cannot transfer it to a person under 21 years of age.
As far as I know S&W have not sold stripped lowers. The lower receiver for the pistol also doesn't have the "buffer tube" and stock, so it would be difficult to attach a stock to it to make a pistol into a rifle with that model. Because of the way the stock is defined, in a standard AR the buffer tube alone without a stock doesn't count as a stock, which is why you can have an AR pistol (the buffer tube is integral in the function of a standard AR). Since the "buffer tube" of the 15-22 isn't required for use of the firearm, I'm not sure if having a "buffer tube" would count as a stock or not on a pistol as it isn't integral to the function.
So it would be very difficult to make a 15-22 pistol into a rifle if you wanted it to be dual duty since it doesn't have the "buffer tube." And it would be difficult to make a 15-22 rifle into a pistol as you would have to make it an NFA item. It would also have to retain the "buffer tube" making it not an exact clone of the 15-22 pistol.
If S&W happened to sell the stripped lowers in pistol form (without "buffer tube"), it would pretty much only be useful as a pistol -- unless someone were to figure out a way to attach a buffer tube and stock with the proper integrity. If they happened to sell stripped lowers in rifle form (with "buffer tube") it would mimic a standard AR pistol/rifle conversion process, so long as you built it as a pistol first.
You were fine until you got to the part about cutting the barrel. It started life as a rifle, so if you cut the barrel it then becomes a Short Barrel Rifle. It must be then registered as such through the BATFE and you must pay a $200 transfer tax. There are other details in between those two steps, but you see where this is going. With the way "things" are going right now, TheLandonMac's $1500 price tag may not be too far out of line! 
