Unauthorized debit order change

On 30/09/2021 I Upgraded my cellphone contract to Uchoose flexi 390 @ R499 pm. I have paid R499 each month since.

Today I received an invoice with the amount R520pm stating that it is Uchoose flexi 410. I went into one of the branches and the gentleman informed me that Vodacom has changed my contract and there is nothing I can do about it as it is within your rights.

I do not agree with this statement. How can they say it is within their rights, to change my contract without any prior notice or communication? What if I can not afford this new tariff?

I have sent them an email 3 days ago where I requested them to tell me where in my contract it states that they can change the amount and package without any notice.

I have not heard anything back from them.