I've seen posts about this problem and it is frustrating when AT&T never really wants to accept that there is something wrong with either how the system works on pro ration or how they explain the details of computation. Either way, it sucks! It is like, AT&T would charge you for the entire billing cycle with your new plan and charge you with the pro rated amount from the start of the billing cycle up to when you upgraded your plan and charge you again of the pro rated amount of the new plan from when you upgraded your plan to the end of the cycle.
Illustration:
New Plan Charge (entire billing cycle) + Pro rated charge of old plan (start of cycle to when you change your plan) + Pro rated charge of the new plan (when you change your plan to end of cycle)
Looking at the bill, yes it is correct that you credited some unused talk and text and data of the old plan. Where did you credit this from? From the new plan? It doesn't make sense to credit the old unused data to the new plan.
Overall, if you scrutinize the bill - how could you be charged more than the new plan amount when you didn't even use the new plan for the entire billing cycle? Doesn't make sense at all!!!
Looking at the complaints and other comments - this has been going on for years now. AT&T has to make a change. Do something about your system or do another round of training to customer service, making them more diverse on explaining things to customer.