As per Cheetah_SA's response, the best way to find out is to do a dummy booking and look at the restrictions.
The full list of Economy fares on SA is (to my understanding and in order from fully flexible and most expensive downwards): YMBKHSQTVLWG. So within one of these 'blocks' (like 'Saver'), there are still different buckets, which explains that sometimes there are different fares within the block for different flights on one day (for Domestic flights). I am not sure where in the bucket list the 'borders' of the different 'blocks' exactly are.
Only YMBK fares are upgradable with Voyager miles. Fares from T downward only earn 50% Voyager miles.