Pro-rata not charged on upgrades?

I just need to rectify on upgrading. I had my upgrading done by a MTN franchise and the lady told me that pro-rata will be worked out on my current upgrade. I was billed the full amount on Sept billing. I phoned in and MTN customer service told me that I would be billed pro-rata on my next bill for Oct hence me short paid the amount. Today I got an email saying that my bill is in arrears (I short paid on the pro-rata) I once again phone MTN customer service and Shaheid Osman told me that on upgrades there is no pro-rata and that the full amount is always billed. This is the first time ever I heard that MTN does not charged pro-rata on upgrades. This is not fair as it is day light ******* for not charging pro-rata on upgrades?