My aunt had a similar dilemma this summer. She was booked in Y as a Gold, on a full flight and wanted to be opipped to J. She had miles to upgrade, but the problem is that you can't use these miles at check-in - you must do it beforehand. Since she can't stand Y, I suggested using the miles, which she did. On board there were free seats in J despite the flight showing J0C0 days before the flight.

Cheers