I think the only way this might happen is if there are IRROPS and BA is willing to be flexible in order to help itself. I had this once at IAD where flights were delayed and when checking in at the desk I was offered the option of the earlier flight as the flight I was booked on was delayed and the inbound hadn't even landed. There was another occasion flying LHR - DUS when I was able to move to an earlier flight which was itself delayed and which should have already departed before I landed in London.
As @
LondonElite says BA is not like, for example, AA where they will happily move you to an earlier flight's Standby List and then you have a fighting chance of getting on board - especially so if you are OWE and effectively move straight to the top of the list.