From what I have read, and the way I drive my '39 is much like driving a truck with a 2 speed read end. Bring the car up to the speed that you would normally shift into the next gear (if there were one), about 35 - 40 mph, pull out the shift knob, then depress the clutch to make the shift. It's designed not to shift until the clutch is depressed. The same is true for downshifting (yes, the car needs to be in motion), but I usually shift out of overdrive around 5 - 10 mph.
I'd be interested if anyone out there feels differently about the above, but this technique has so far served me well, knock on wood. Enjoy the car. columbiatwospeedparts.com is wealth of information about the Columbia and it's proper maintenance.