Billed "pro rata" for 1 day after an upgrade at a rate of double the monthly rate.

Got an sms early in April that I'm due for an upgrade. Went to the Somerset Mall branch and upgraded and got the new phone immediately. New contract only came into effect on 1st August. My August bill reflected a charge for the postpaid account for the period 30 to 31 July for almost R500. My monthly subscription is around R260. I've been into the store and called the call centre to query this charge and have got tons of different answers including that it is a pro rata charge and a settlement balance on my old handset??? My July bill states it is for the period 1st to 31st July and that was paid in full and the settlement on my handset was R0.