BA will accomodate free of charge for same day changes on flights with Club Euriope + Economy as long as the fare that allows for baggage and same day change is permitted - on the HBO fares it will not

On other flights, it will allow changes as per fare rules only

AA will allow you to make free change changes as per fare rules - many of its fares allow for changes free of charge

I'm not sure how AA's confirmed flight change and syandby are restrictive - it allows change to an earlier flight on the same route as existing - CFC allows you to confirm before even heading to rhe airport

Extending to another carrirer - Qantas, for example, on its domestic fares, discount fares allow changes at $100 until day before travvel, but no changes at all on the day - arrive early and enjoy the airport longer - arrive late and buy a new ticket