Paid seat, flight change

[tontoro]

If you pay for a seat, then at a later date change your flight to a different day, what happens?

[John]

https://www.britishairways.com/en-gb/information/seating/seating-changes-and-refunds

You can use the money you paid to choose a seat on the new flight, but no refunds if it costs less

[tontoro]

Thank you