Air India review: regarding money refund as downgraded from business to economy class

I booked Air India flight from Delhi to Bhopal for 11th January 2020 at 5:55 AM,
Business Class and I paid 16, 139 INR.
When I reached airport on 11th Jan, I was told that due to some technical issues the flight was changed, and the flight which is leaving at 5:55 AM does not have any business class.

They issued me a seat in the economy class and said that the money will be refunded after deducting the economy class amount.
I made the booking online on Air India website through internet banking (State bank of India).

The given economy seat details are below:
Boarding time- 5:16
Boarding Number - 44
Gate : 42 C
Seat 1F
Flight - AI 435

The Business class ticket details are below:

Ticket Number - 098-[protected]
Web Reference - AIBE56759953
PNR - YF950 (AI)
Flight - AI 435
Class - Business - Z
NVB - Sat, 11 Jan 2020

I have emailed them multiple times and called them, but no body responds on either medium.

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