the Pentax 645 I use doesn't have interchangeable backs, unfortunately. Meaning I'll have to finish a roll before I start a new one.
The reason I don't shoot ISO 400 film at 640 is to get a bit more "deepness" on the film. I've shot Fuji 400H at 640 ISO and the pictures turn out almost "faded". Maybe I'm metering wrong? Anyways, I figure if I shot at 640, the picture would look even more faded and wouldn't have that 'look' I'm going for.
I may do something along the lines of shooting higher ISO film in one body and my main 400 ISO on a lower body, although using 800 speed film and shooting at 400 sounds interesting.