It can differ if you took a promo package last time ie, took a phone that you had to pay off on your contract. This happened to me. My upgrade date was the 26th Feb but my promo date end was 20th March.
I would call their callcentre and speak to an agent, the MTNSP store also thought I had to wait until my contract termination date but the callcentre didn't. Get a reference number and the persons name you speak to.