I am trying to book a round trip award flight from ORD-BKK and seem like on the return flight most portion from BKK to NRT or HKG are only on economy class but the other portion to ORD would be business. If I book now and then business class opens up on the first portion. Will there be a fee if I request for the change? Also if I end up with economy class on the first portion, what my luggage allowance will be? Economy class allowance or business?

Thanks