In the end the other guy got lucky and you (OP) jumped the gun.

It is a crap situation but it is what it is, on the spot they found a way to help (which was too late for you) but once you made the change they had no responsibility or option to help you anymore, especially since you probably want them to pay you the difference. That answer will always be no.

Lesson learned, never decide to change a ticket until you are about to board. Consider if booking a new ticket once you land would have been that much more expensive. You could have tried it with BA after landing as well and plead with them that your delay was so massive you could have never seen this coming. They might have been able to make the change or offer you the same change fees as you saw before.

Globalist