You will have to do an airlaw and procedures exam and pass a flight test with a CAA examiner. They will then only allow one renewal after which you will have to write three exams in order to get you licence converted into a full SA one. Try calling Claudia in licencing +27 11 545 1000 (they take a while to answer) and she can give you more gen.
The validation is dependant on you foreign licence being current and they will only allow you to fly aircraft that you already have on your current licence. But otherwise the procedure is fairly painless. Good luck!