Status (whether BA or OW) is entirely irrelevant to flight change options on BA. There is no same day change beyond fare rules, ie

- for long haul and for any connecting itinerary, whether you change on the day or any other time will always be whatever change fee and fare difference regardless of status (bar exceptional circumstances such as overbooked flight, major delay expected, etc)

- for short haul point to point only, some fares include same day free flight change as part of the fare rules, if you have one of those, you will be able to change on an earlier flight (regardless of status) and otherwise it will be as above, change fee + fare difference (again, regardless of status).