From the Guide:
While A&M is essentially the same as a standard redemption, we've highlighted A&M in its own section due to a few unique restrictions that exist with these tickets. A&M booking are more restricted than standard redemptions. No changes to either route or carrier airline are possible after the A&M booking is made.
You may however cancel an A&M booking and receive a refund of both the Avios and cash components so long as you cancel more than 24 hours before the departure of your outbound flight. If you cancel within 24 hours of departure you will forfeit both components paid.
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