Hi Brooke,
The rule of thumb is when you go down in band size you need to go up in cup size and ice versa. In your example, if you are a 44C and want to try a 42 band size, then you would need to go with a D cup, since 44C and 42D have the same absolute volume per cup. In the industry they call that 'sister sizing', i.e. 46B and 42B are both sister sizes of 44C.
Hope this makes sense.