BA will re-open those seats and re-allocate / re-sell / accommodate new customers changing flights / accommodate operationally necessary upgrades etc. as they see fit
My wife once had to cancel a trip with me at very short notice (she was due to join for a work event) - her (former) Club World seat was occupied by someone else when I got on board, even though we had only called through the cancellation whilst I was heading to the airport less than 2 hours before. BA won't hang around with this.
Bad luck...