No, the voluntary cancellation rights on Avios booking apply to all airlines, not just BA operated. All redemptions can be cancelled at 24 hours notice with a £35 fee maxima, with no reason being required. It starnds alone and doesn't go away after rebooking (etc). This is different to other frequent flyer schemes, and even Avios redemptions on Iberia, for example. The involuntary rebooking for a cancelled flight bring a separate set of policies, as you seem to know. If there is space in the replacement flights, there is an argument to hold off on the rebooking since you have more leverage at this point, rather than rushing to whatever option is currently available.