air transport – flight rescheduling fees for Indigo?

In Indigo, there is a Flex option that I do not know what it represents.

Apparently, this allows you to reprogram the flights. But I was told at the door that the full fare would be required to reprogram the flight, which is confusing.

To take an example –

Let's say I have a flight on December 10, 2018 from Cochin to Pune (COK to Pune) 6E 337. I have already booked the flight to say 3000 INR / – which includes the Flex option (1 000 INR). See the charges

I now wish to prepare for my trip on December 3, 2018. It has been said that I should perhaps again bring full charges, which seems an absurd suggestion. This happened on a recent flight and I had to buy a new plane ticket, then come back and cancel the flight.

Just for reference, I saw this news on the same subject 3 years ago.