ExXB
Yes I (we) have retained our boarding cards for both flights and they do show different flight numbers. The original flight is 6xxx and the "re-scheduled" flight 9xxx.
Surely the simple question is "Did flight 6xxx take off?". The answer is no.
I also understand that "technical reasons" is not classed as "exceptional circumstances" under which the loco can avoid compensation.
However, having spoken to the AUC it seems that they are most interested in progressing this. We shall see what transpires.