I am installing copper pipe to a new bathroom. The pipe will be an
extension from the existing galvinized pipe in the house. The distances
are not great (10'-20' range)

The existing cold supply pipe is 3/4" everywhere but the hot water lines are
only 1/2".

I want to install 3/4" CU pipe for both hot and cold (reducing to 1/2" just
before the fixtures) in the new installation in anticipation of repiping the
whole house in the future.

This plan requires that I transition the hot from 1/2" galv to 3/4" CU for
the hot water. My question is "will this create a pressure or flow rate
problem?" Normally you go from big to small pipe but in this case it is
from small to big.

Should I keep to my current plan or change to 1/2" CU for the hot all the
way through?