This is somewhat complicated. But I believe the way it is, is that you can call for the $50 change at the 24 hour mark from your original flight, but no sooner than 12 hours before the departure of the new flight. So if you call - 12 hours before the Sunday night flight - which is likely in the 24 hour window - you might be OK.

This is confusing. Others please chime in. I think you should be able to call for change at 9:45am on Sunday am - which is 24 hours before, and within 12 hours of the Sunday PM flight.