I suppose you would request the fare difference -- not 'downgrade compensation' -- from UA, although I suspect it'll be a few dollars maximum.

ETA: On second thought, it depends how it was fared. If it was a through fare to WDH, which was my assumption, then I stand by my statement. If it was a separate fare, then you should get whatever the difference is between J and Y on the JNB-WDH sector.