Assuming the cars are as modular as the older Kato offerings I have in my collection, trading out for a coach interior from the Kato parts department ( or one 3D printed one if you want all the seats in exactly the right place) should be simple. The interior is a separate part, and all that might be needed would be some minor surgery on the new interior piece to get notches in the right place to fit, and possibly relocating a bathroom partition.
Another possibility is the Con-cor Budd coach that is modeled on the CZ cars after conversion. (Con cor made 2 different Budd coaches, the second is an ATSF chair car prototype, which they market as a 'twin-window coach'.) For that matter, it might be practical to install the Con-cor interior into the Kato carbody (these cars were a major upgrade from old Con Cor passenger cars, and have separate and reasonably accurate interiors, similar to Kato). I don't know if Con-cor will supply parts any more (to get an interior), haven't purchased anything from them for years.
I'll admit I didn't think about the case of swapping out the interiors for the Kato cars. The Con-cor interior into a Kato carbody is something rather interesting I'll have to check out and probably try and acquire a sample one to see how it'd go.
