I did just that when the language proficiency came in a few years back. There are certain requirements, like you have to have attained a certain grade in English first language at high school, I think up to a D. If you took English as a second language then the criteria in terms of grades is slightly higher.
Then it's just a matter of going to the CAA with a certified copy of your matric certificate, and writing an affidavit stating you have no speech impediment and they should issue you with a level six.