I’ve had CE cancellations resulting in downgrades on the rebooked flight more than once. On one occasion (Avios flight) the refund proffered eventually was less than the difference between ET and CE when I was originally booking with no explanation offered. On the second occasion, I had taken a £69 POUG, which one would think would be easy to reverse. It took three or four rounds of back and forth to extract a refund.
On one of these occasions, CE availability on the rebooked flight was coming and going. The customer service desk in Galleries Club were keen to help and made some calls but no luck.