Not sure for BA based ticket, but for AM based ones, you can definitely change anything before travel commencement without extra charge, with the following exception that will cause re-issue

1. Change outbound segments to earlier days (same day does not affect)
2. Change outbound and / or inbound segment which are later than the origional expiry of ticket

I change my return day 3 times before travel commencement, all within ticket validility therefore no charges. For award ticekt think it as a full Y ticket with restricted avaliability.