Vodafone — charged twice for same service

I had activated vodafone mobile connect activated in my mobile phone with 499 plan.
after using if for some time last month i changed my plan to 699 plan.
i called up call center and was asked to deactivated 499 plan and then activate 699 plan.
i had used 499 plan for 6-7 days only.
but when the bill was generated i was charged for both the plan ie 499+699 rather than being charged only for the number of days i had used both the plan.