I had a similar scenario a couple of years ago, JNB-CPT on SAA, where the virtually hourly shuttle service has a few A340 operated flights. My A340 flight was changed to B737/A320. I was able to negotiate a free change back onto the A340 operated service, which was now running an hour earlier than the original plan. I can't recall if this required a couple of calls or went through first time, but as always, if at first you don't succeed .....