You won't be able to upgrade unless you are a Skywards member and have the required points.

I think the only way you could do it would be to sign up to Skywards, and then transfer your Amex points (assuming you can do this).

Note that the DXB-JNB/CPT flights will be busy during the world cup, so it's doubtful there will be any room to upgrade into.

Cheers,
Rick