Restricted fare, earlier flight: options?

Due to a schedule change in the oncoming DL flight, one of the next Sunday we should arrive in LHR 1hr earlier than planned (11am). Then we have a complete separate BA ticket - CE cabin, I fare class - on last flight to BRU (18.20pm).

Is there any option to take the earlier 15.30pm BA flight without throwing away the existent return ticket and buy a new o/w one?

Thank you! :)