On BA all business class tickets are changable for a nominal fee (about 5% of the cost of a return ticket). It's that flexibility that means people pay £6k return for the cheapest business class LON-NY ticket.
Sure, if you're flying on the "cheapest flight on skyscanner" then you don't get the flexibility, but that's why they don't fly 6 A380s rather than 12 smaller planes, they want the £6k tickets to choose BA/AA rather than VS/DL. The £600 return tickets at the back will fly whoever's cheapest.
I looked at a selection of business class fares (including fares booking into I buckets) a couple of weeks in advance and all were changeable for £300.
"Almost all" then, especially when you factor in corporate deals
Assuming you have a flexible ticket, which the overwhelming majority of passengers don't.
Tried asking BA/AA to move to an earlier flight on a non-flex ticket? "Computer says no, unless you pay $lots"