NO UPGRADE ALLOWED AFTER PLUS 8YRS

I was on a contract for 24 month paying R400, my contract ended and I opted to go on a month to month contract until I saw I nice deal that I wanted to upgrade to, I called cell c to upgrade and only to be told I dont qualify for a R200 a month contract and yet I am on a R200 month to month cost, i asked the reasons n was told there was money which was not paid on my contract 3years ago. very disappointed and bearing in mind they are the ones who were debiting on my account so if there were unpaid funds on my 3yrs ago contract how did I qualify for the one that ended last month, very disappointing and i am going to log a complain vie the ombudsman, i even cancelled my month to month contract cos clearly I am not valued...