Shocking underhanded charges without knowing from Vodacom!!!!

I've been on contract with vodacom for the past 8years, this year being my fourth upgrade. Everytime I get a notification from vodacom stating I can upgrade earlier than the end of my contract date and NEVER DID THIS RESULT IN EXTRA PHONE CHARGES. Suddenly I get an invoice that for the next two month my contract amount would be almost double to cover the previous phones payment or something in that line. THIS WAS NEVER MENTIONED OR DISCUSSED WITH ME DURING THE UPGRADE PROCESS. I feel I should cancel my upgrade, cancel my contract when the time ends and move to another service provider. Absolutely shocking from vodacom to come with this underhanded additional charges without disclosing it to their customers.