Originally Posted by orbitmic

There are lots of threads asking that question - often started by US based flyers used to US airlines' practice. The bottom line is that there is no equivalent for BA* or for that matter other European airlines - no space available same day flight change or same day flight standby, and if you want to change flight - be it in advance or on the day, it will typically imply a change fee and paying difference in fare.

As others have mentioned, there can be exception - for instance in case of significant disruption or significantly overbooked flight, but by very, very far the most likely answers are "no" or "sure, it will cost x".

For that reason, if you are arriving at the airport anyway, you can always ask but expect those negative outcomes to be by far the most likely. However, I would never recommend that you go to the airport too early just to ask about taking an earlier flight if you don't have to be there early since the overwhelming probability is that you'll just wait far longer than anyone wants to at the airport.

* The very specific exception on BA is for short haul BA flights when booked as o/d one way or return (ie no connecting flight, no open jaw) in either economy plus or business class. There is no equivalent for long haul or connecting flights and fare rules thus apply.