My experience
I actually needed to upgrade my 192kbps to 384kbps anyway, and will probably need to go to 512kbps sometime soon. After reading Debbie2's posts I decided to start the process now, instead of waiting for them to do the 'free upgrade'.
So I phoned this morning at 8:05. Was quite surprised when it was answered within a minute or two. And it took only 15 minutes do finish the call - my normal waiting time for the repair side of DSL to answer. And the lady was very helpful too.
We managed to establish the following:
1) Yes, my exchange supports speeds higher than 192 (not all can, apparently

)
2) Yes, there is an imminent "upgrade everyone to 384 coming" (but what about point 1, then?)
3) The reason for me doing this (Yes, I had to supply one)
4) That I am not a Telkom Internet subscriber. And I don't want to be. And own my own modem.
5) It is not entirely clear to her if a technician will need to come out to my residence. She suspects not, that he will only need to install software at the exchange for me.
6) That it will take them 30 working days to do. Which puts us in the middle of July (Someone needs a new calendar).
All-in-all it may have been better to wait until August. According to her everyone will be upgraded on August 1st. After my phonecall today I somehow doubt that it will happen overnight. After all, all those techs have to install software for thousands of users in a single day ...