I'm afraid that with flights using miles/Avios it is difficult on many routes to be picky about what specific dates you want. If I were you I would have booked the flights I thought I'd want and pay the fee to cancel if I needed to (and there is no fee for Gold members).

If you can't change the dates, can you change the airport? Seatle and Vancouver aren't too far apart and MrsLA and I had a very pleasant two centre holiday a few years ago.....

If you were a Gold BAEC member it may be worth calling up, but otherwise I think they are unlikely to release any additional seats.....