Airtel — Postpaid billing issue march

Address:

2/64A, THORANA MALAI ROAD KANAVOOR, MATHAPURAM POST, KADAYAM

I have been using the airtel postpaid 999 plan since long time and suddenly my billing amount being increased 300 while enquire customer care about this they were clarify that the plan has been revised and you will be charged extra 300 every month. I was really surprised and how could they change my plan without mu knowledge.