Mtn keep changing their policies

I upgraded my handset on 3 August, 2015 and am not happy with the scheme which is a My Choice 200 Top-Up contract at R619pm with only R200pm for airtime. I was told I could amend my contract after 3 months if I wanted to by Lindani Moyo, formerly of the Killarney Branch. 3 Months have now passed and yesterday I went to Eastgate branch to amend my contract and I was told I would have to \Pay in R7000"to change my contract because it doesn't allow me to change due to the plan being a \""Promotional Offer\"" and nobody had explained this to me properly at Killarney Branch. I am confused because the plan I want to switch to is R639pm and included 500mb data and an SMS bundle as well as R200 airtime. I think it highly unfair that I can't change my contract because this wasn't explained to me properly. I think it's time I went to the ombudsman. Why should I suffer the consequences for your company's rules not being explained properly"