Delta does not allow for SDC on international travel. You would have to pay the change fee plus the fare difference.

They may not have any upgrade space available on the flight. If you re-book it would most likely be into Y. Some GAs will keep you in J but technically your reservation is in Y so that is all they have to re-book you.

If there is a IRROP then you can change for no charge. Otherwise it is not allowed without the fee and fare difference.