Splitting up a circuit

I would like to split off about half of a 15-amp circuit and add it to a 20-amp circuit.



The 15-amp currently has the garage, a bank of kitchen lights, a full bathroom and the master bedroom on it. I would like to split the bathroom and master bedroom off and add them to the 20-amp circuit which only powers a small full bathroom [which has an additional 20-amp going to it].



My questions are: Is there an obvious reason not to do this? and, If I do it, do I need to upgrade any of the wiring?



Thanks





edit: average-to-light loads in bathroom and master

Splitting up a circuit