And when do you decide that? A waitlisted upgrade on an actual AA flight, even if comes, might come in each direction until you're at the gate, shortly before boarding! That means you might not know by the time you board the outbound whether you'll get upgraded on the return (since you're not returning at the same as you're going outbound, of course).

But to make it simpler: You're guaranteed to not get upgraded to business on the LHR-JNB legs, for multiple reasons: Because it's BA, and you can't upgrade at all with AA miles on an O fare on BA; because it's BA, and you can't upgrade from economy to business on BA from any fare at all (only to premium economy, which BA calls World Traveler Plus or WT+).

So if you know right now you can't possibly be upgraded on LHR-JNB, and that's live-or-die for you, you might as well cancel the ticket right now. No point in waiting for something that can never happen.

Obviously, in retrospect, I think you see that you have researched all this before you bought the tickets. But as I surmise from your TripAdvisor thread, you didn't know about FlyerTalk before someone there referred you to it.

What you may not know is that there are other carriers who may be able to get you from MIA to JNB on an outright business award, that don't show up on aa.com, such as Etihad. (Though obviously, you need more miles for an outright business award than for upgrade awards, and I don't know if you have enough miles.) Another thing you might not know is that it may be easier to book awards about 11 months out (when the flights first appear in the schedules) than anytime later. Anything thing you might not know is that it may be harder to find 2 seats on an award (or for that matter, upgrade) than just 1 seat (so you need to careful when evaluating other people success' stories to know whether they were traveling solo or as a pair).

That's why you need to learn a lot before you'll necessarily know everything you know to get this trip to happen all in business class (in a way that you can afford, assuming you can't afford to just buy business class seats right now). EIther that, or you need to (a) have more than enough miles for whatever the solution might be and (b) hire a professional mileage award search outfit to find you flights that would work for you. (I know such services exist because I see them mentioned on blogs. I don't know how many people on FlyerTalk use them, because I rarely see them mentioned in FlyerTalk posts.)