Disappointed after 13 years of loyalty...

My contract term has it's anniversary in February. I received an SMS to say i was elible to upgrade so i contacted a store and chose a handset etc. This was in December - 3 months before my usual date. I didn't think much of it because i have upgraded this line 3 months early before so i didn't foresee any issues. When i got to the store they hit me with paperwork to say i still need to pay the balance of my old contract until it's anniversary in conjunction with my new contract. This was never explained to me at any point and i feel that Vodacom leads you to believe you can upgrade but don't tell you the hidden costs. I assume this is a new thing as i have never been told to pay the balance of my contract?? By sending a misleading sms i feel that it is not treating the customer fairly.