You are absolutely entitled to compensation; the question is how much compensation.
This falls under the rules of SA's terms and conditions:
10.2.4 If we are unable to provide previously confirmed space, we shall provide compensation to those Passengers denied boarding or involuntarily downgraded in accordance with applicable law and our denied boarding compensation policy.
Unfortunately, I found nothing in the terms and conditions that talked about how much compensation is involved for an involuntary downgrade. (The T&Cs go on to talk only about "If we cancel a flight, fail to operate a flight reasonably according to schedule, fail to stop at your destination or Stopover, or cause you to miss a connecting flight on which you hold a reservation...")
Because you had already begun your trip, I do not believe that it matters whether you were holding a RTW ticket, a full fare ticket, or a FF award ticket. I believe that they owe you the difference between full-fare Y and full-fare C. According to EF, full-fare C is ZAR 21540, full-fare Y is 10950, so I believe you have a right to claim ZAR 10590 (or about US$1325).