e.g. let's say we have a signal with B = 100 Hz that is sampled at 400 Hz. The anti-aliasing filter will remove replicas, and there is nothing to alias, and we downsample. If we downsample by 2, we will get signal of 200 Hz. But what's stopping us? Can we downsample by 4, or 8?
(why can't we downsample by 4, get B = 400Hz, then have a new sampling rate at 800 Hz?)
Similarly, is there a limit to how much we want to upsample?