I have a different experience that may shed some lights.
I flew HKG --> CTS --> HND // NRT --> HKG in summer with redemption J booking, while there's no J for CTS --> HND, I sit in Y but the weight limit is still 30 KG
while my friend, on redemption Y, flew HKG --> HND --> CTS --> HND // NRT --> HKG, he got 20KG all the way as well (which for JAL domestic Y baggage limit is 10KG or 15KG)