There is no DeviceMono color space in a PDF, so it is the BPC 1 that separates a mono image from a gray scale image. In both cases the number of channels is 1. If an image has more than 1 channel, then it will be treated as a Color Image.
Mono : 1 Channel 1 BPC Gray : 1 Channel Color: 2+ Channels
Changing the BPC of the image would almost certainly “break” the image, causing it not to render correctly.