Charged double after contract upgrade

I did an upgrade on one of my contracts effectively for august. Under the old contract the last bill i received was for 1 Aug till 31 Aug. My upgrade was processed on 13 Aug. When my September bill came I was billed for the pro-rata charges from 13 Aug till 31 Aug as well as 1 September till 30 September. Normally a disconnection credit would be app**** since you cant pay for the same month twice. My issues with the charges is the following. Original charges 1 Aug till 31 Aug should be credited so that I only pay for 1 Aug till 12 Aug. Prorata charges for 13 Aug till 31 Aug is correct. Charges for 1 Sep till 30 Sep is correct. Cellc does not want to give me the disconnection credit.