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Did you check the fare difference? At the gate would be too late and, while there is no fee to change, you would be charged the fare difference for the week-earlier flight. The IndiGo Flexi Fare Terms and Conditions outline the fare offer (added emphasis mine):
Our all new Flexi Fares are attractive providing flexibility to make unlimited changes to travel dates without incurring Change Fee. This Fare is just right for the customer who wants flexibility and comfort in their travel.
- Unlimited changes can be made to a booking where there will be no change fee however, the applicable fare difference would be charged
- Cancellation Fees for Domestic travel is Rs 3000 and Rs 4500 for International travel
- Changes can be made to a booking upto 2 hours prior for domestic flights and 4 hours prior for international flights