A few things here as I believe both responses above are not wholly accurate.

First, EC261 may not apply because they have given you ample time to re-arrange things. Generally EC261 (especially for cancellations) is not applicable if the carrier alerts you at least 14 days ahead of your flight. In this case you've been notified 6 months ahead. But I am not sure if the 14 day rule applies to downgrades. So continue reading...
Second, you have more options than "athome" gave above. Yes, you can take the flight as they have rebooked you or you can try to rebook on a date where SA is flying an aircraft with business class available. You can also get a full refund of your ticket which wasn't mentioned above.
Third, downgrade comp is going to be 30% of the leg for JNB-VFA since that is the affected segment. Downgrade comp only applies to the segment(s) that were downgraded and not to the entire ticket. So the fact that you are starting in Europe doesn't matter for calculating distance.

You should either take the flights as they're now booked (Y on the SA leg) or ask M&M if they can rebook you for a day that has business all the way through. I'd expect SA to continue to change their plane assignments so it's possible you get bumped out of business again, if M&M is even willing to rebook you. Will depend on whether they can find the same mileage rate for new dates.

At the end of the day, I don't see this going anywhere for you with regards to compensation. LX/LH, whoever ticketed the reservation, doesn't have any EC261 claim against it as EC261 is based on the operating carrier causing the issue, not the ticketing carrier. And if you call SA and demand downgrade comp for JNB-VFA they're likely to calculate that segment of the award as "nearly zero" and offer you nothing.

As "athome" correctly pointed out, you would likely have paid 60K points whether it was initially booked as business all the way or business/coach as it's now booked. The long haul segment makes it a 60K award for business.

Please keep us posted on what you decide to do. LH/LX should at least try to help you rectify the situation by rebooking.

-RM