Mytrip.com complaint: Delay in changing ticket booking

I booked on 27th of October a trip from Venice to Lisbon (Order LVDCEB with flexibility) for 21st of November 2022. After 34 days I called to move my departure date one day earlier (20th of November). First of all, there is not any online rebooking system. I had to call their number in USA (from Europe!). Once I called, explaining my request, the operator told me that I would have been contacted within 2448 hours . Everybody knows that rebooking with flexibility is subject to the payment of the price difference between the tickets. At that time the difference was around 10€, after 12 days the price to be paid would have been around 40-50€.
Nobody called me. I recalled yesterday (07th November) to ask again for changing my booking, The operator told me that the difference between the tickets to be paid is 245 $! This is unacceptable! Nobody considered my request at the moment when the difference was way cheaper.