Bottom line is that you are due a refund of the fare difference. That may not be much, but it is due to you as a refund.

I would not do anything until this trip is completed as the last thing you want to have happen is for the ticket to get loused up.

When you get to the counter, ask whether by chance a seat has opened in J and then ask again at the gate. If there is no seat, just fly the assigned seat and deal with it after arrival.

Hopefully this will not happen again, but when problems arise, it is best not to wait to be given a reason. Rather, one makes polite but firm inquiries. Sometimes one gets accommodated.