It would likely vary wildly based on your status with SAA, the class you're in, the fare you paid and the complete whim and caprice of the person(s) handling your rebooking. I've been rebooked from IAD to JNB on a given evening to a JFK to JNB the next morning in a coach seat purchased with (IIRC) DL frequent flier miles by a "nice" agent at IAD. I've been booked on paid seats from JNB to CPT and pleaded for a last minute change at the airport and been told to pound sand.