Yes, they could do exactly that if they really wanted to. Of course one would have to question throwing away the existing modules in favor of the new ones. And in the case of the dining cars, with the special emergency doors, it would be hard to covert them into a sleeping car. You'd have to put some metal plates over the doorways first. And baggage cars also pose a problem, with the loading doors. The easiest switch would be coach to sleeper or sleeper to coach. But again, yes, the theory is correct.
In fact, were funding to be found, Amtrak could even use this modular nature as an easy way to refurbish the existing 50 Viewliner cars. When Amtrak was busy with the program to rebuild/refurbish the Superliner I sleepers, it took an average of 3 months per car to do. This was due to the intense nature of the work needed.
If I had to hazard a guess, Beech Grove could probably refurbish a Viewliner (at least the rooms) in 3 to 4 weeks. Simply open up the side panel, unbolt/unhook the current rooms and slid them out, then slid in new rooms bolt them down and hook up the utilities and you're basically done. If they want to redo the AC or the trucks, it might take a bit longer, but still in a month to a month and a half you'd have a fully refurbished car.
And it would cost considerably less than the traditional method and see the car out of service for far less time.