If you mean your data product, then yes, that is easy enough. If you mean your line speed, it depends entirely on Telkom's available resources on your exchange. If there are limited ports on the exchange, you may not be able to upgrade, or downgrade back, as Telkom will usually have waiting lists for ADSL. It's possible in theory, but I would be hesitant to recommend it.
If you're currently on 2Mbps, you'd only be paying less than 50% more to increase your speed by 100%. I think it's worth it just to go for 4Mbps and stay there. Barring any issues on Telkom's side, it's very difficult (emotionally) to go back to slower speeds