Not changing the speed after you have cocked the shutter is a general rule of thumb for both large format and older medium format shutters such as the one on your folder. I am not familiar with the inner workings of your particular model. The point is not that you will get another unknown speed, but that you may damage the mechanism. It may not happen the first time and you may not notice a slight change, but it is an older mechanical device, more likely to break when used improperly than something newer.