Given lower limit for my contract upgrade

I went to Cell C at Secunda yesterday to upgrade my contract. My current contract is Straight up 200 and I am paying Cell C a total of about R600 a month. I wanted to upgrade to Infinity Select Sim Only for R999 and I was told that I only qualify for R300. This is very embarrassing because I can afford the R999 and they told me that I have been given R300 because of my bad payment history. I have never even missed a single installment and my credit score with Transunion is 989. I have been a loyal Cell C customer for years but our relationship with Cell C ends here. I am going to cancel all of my contracts with them.