No short answer, sorry.
Scenario 1: No clause allowing them to change prices - they can't change prices, you can't cancel. If they try to change prices, they've breached the contract and you can cancel immediately, without penalties, if it's deemed that the breach is material (my opinion: in this case, it is material - price is always material).
Scenario 2: Clause in the contract that allows them to change prices, but doesn't allow you to cancel if they do - such clause is illegal in terms of Regulation 44 (3) (h) (issued in terms of Section 120 (d) of the CPA), and void in terms of Section 51 (3) of the CPA, so basically the same outcome as scenario 1.
Scenario 3: Clause in the contract that allows them to change prices and allows you to cancel if they do - penalties are determined by the wording of the contract. If they're vague, they're illegal in terms of Section 22 (2) (a), and also void in terms of the common law, so no penalties. If they're excessive or unreasonable, they're illegal
So basically, the only scenario where they can charge penalties is if the price change clause is complete and legal, and the penalties clause is complete and legal (meaning reasonable). Highly unlikely, unless Vodacon's contracts have changed drastically in the past few years.
My guess: Scenario 2
My guess on the outcome of this whole thing: Vodacon doesn't care - if you cancel they'll illegally report you to the credit bureaux, leaving you to sue them and the credit bureaux; if you complain, they'll ignore your complaint and bill you the new rates anyway; if you sue, they will sink you with legal costs over the next decade (if you last that long), your bank balance will drop rapidly, your stress levels will rise rapidly and everyone you love will leave you, your health will decline and eventually you'll die, broke and alone - basically, they'll slowly murder you. If you complain to the NCC, nothing, since the NCC isn't even responding to complaints any more - Vodacon has already murdered them with litigation.
Good luck, and switch to prepaid ASAP.