Just a thought, but will the "glass" from an older model (that fits on your chassis) fit in the newer shells that you want to use?
I ask because, although Kato is not Atlas, new Kato F unit shells (for the DCC chassis) can be used on old non-DCC chassis by swapping the "glass". (also works for putting an old shell on a new chassis)
This is not to say that Kato and Atlas went the same direction with their newer chassis, but certainly seems possible.