Well as long as the interchangeable Caliber only means going from the 45 ACP to possibly the 9mm NATO it probably shouldn't be that difficult.
I mean all you really got to do is design a bolt and barrel that can be swaped in and out that is the same external diamentions execpt the barrel would be chambered for the smaller 9mm and the bolt will be suited to hold the 9mm case head. The difficulty in my opinion would come into play with the magazines, might take alittle fine tuning to get a magazine that not only would you be able to feed 45 ACP in but have another magazine that will have the same external diamentions that can feed 9mm. Which would make it almost like a "double stack" magazine in 9mm but probably a single stack in 45 ACP.
I'd figure since it would be able to hold the 45ACP you can probably get it to work with minimal difficultly (agian execpt for working with the magazines fallower perhaps to provide good feeding) with:
If only I had the ability to make what I'm thinking about exactly in my head on in a real working prototype I think I can get it to work better then the 5,000 MRBS and 10,000 MRBF they want. Might end up being alittle expensive for the military though at about 750$ each (tooling costs for a small number would be a killer could probably sell them on the commercial market as well to offset costs).
Seeing as the M9 Barretta has a replacement cost of 263$ I guess they are ripping people off when they sell them on the commercial market.