Cancellation fees charged when updgrading to remain a client.

On 21 May Webafrica sent me a bill for a Clawback fee of R999 which in their customer agreement is charged when a customer cancels with themselves. I merely upgraded my line and have been a loyal customer for 5 years now. How can a business charge a client a penalty for merely upgrading a service and chosing to remain a customer? This is contradictory to a cancellation policy.