You should be able to change lenses mid-roll in ANY Mamiya tlr.
To change lenses,you need to turn a dial to the "unlock" position,which moves(if the camera is in full working order,and the seals are ok) a light baffle between the lens and the film.
I have a C33,and despite the extra weight and the lack of a built in light meter(I have a reasonably priced handheld meter which weighs next to nothing,so that's no great problem!),I prefer using it to my 35mm slr.
Basically,it's a simple camera-which makes it easy to use.