Trip.com Charged Me Nearly Full Fare as Change Fee for My Return Flight

I tried to change the date of my return flight on 11 September. The system is asking me to pay €420 as a “change fee,” which is almost the price of a new ticket. I have already paid for my ticket—why am I being charged an amount comparable to a full fare again? This is unfair and misleading. Please provide a clear breakdown (penalty + fare difference), correct this excessive charge, and resolve my issue. Booking/PNR: 163831498101825971 Reference: 89266478.