Vodafone India — does not attending the plan change request

As on 21 st oct 2017, i reuested the customer care to change the plan from 699 red to 124 simple calling plan. After 10 days when i trying to pay bill i saw the my reuest has not attended and now they are reuested to change the plan now. And make their bill on prorata basis. Why should i pay the bill at vodafone fault.
They did not inform me as we change the plan or not.

My moile no. [protected]