There are some options open to you during the year. Whilst it is correct that you have unrestricted plan change opportunities at year-end, you can still change some plans during the year. You need to talk to a professional broker to investigate for you.
Yeah you can, I've switched from Discovery Classic Saver to Discovery Classic Delta Saver mid-year ... ended up saving myself a few hundred at a time when I really needed that. The thing is I could make that switch because it didn't affect my actual risk level to Discovery: the 'Delta' restriction was just a limitation imposed by myself on the number of hospitals I could use, and the penalty fee I'd pay if I went anywhere else. All the actual medical coverage remained the same, which is why Discovery didn't mind me making that switch.
The reason they only allow you to do it once a year is because I think they plan the risk pools and premium levels annually ... it's also so that you can't conveniently increase your medical aid eight months into your pregnancy (for example) just so you qualify for increased benefits over only this period, and then drop down immediately afterwards.
With health insurance you seriously need to take the long-term view: do I want to pay a premium for the rest of my adult life to a company in the bet that when I fall sick I'm going to need a lot more money in one bulk payment (e.g. cancer diagnosis or serious vehicle accident - both quite likely scenarios in South Africa) from them than I'm likely to ever pay them in these reasonable installments. It is NOT something you do on a month-to-month basis, because **** can strike at any time.