12 months to 6 months change

I went into the Woodlands Edgars store to change my account term from 12 months to 6 months interest free. I was told by the gentleman who helped me that he cannot do it; I must phone the number on the back of my card. The lady on the other end of the phone proceeded to tell me that I cannot change my term from 12 to 6 months. I said to her I specifically asked when I opened the account if I could change to 6 months and was told yes I could. Now I have to stay on the 12 months even though I don't want to? I feel I am being deliberately taken advantage of as a consumer, I went through the terms and conditions on the Edgars website and I could not find the terms or condition that you cannot change from 12 to 6 months; if it is there I did not see it.