I, too, found out about that seasonal equipment change JNB/HKG and changed my plans as a result. Also, with ticket prices now much cheaper in South Korea (for AONEx tix), I'm no longer going to JNB/CPT anyway. (Used to do MRU before that.) If you want to fly two days later, you might try just calling BA to make the change. They might not notice the one year problem, and, in my experience, once they agree to a ticketing change, they'll honor it, even if they subsequently discover that the change violates a rule.