Originally Posted by Infinite Possibilities

This question probably belongs in the EC261 thread: The 2018 BA compensation thread: Your guide to Regulation EC261/2004. In particular post #5 in that thread deals with downgrades and may cast some light on this.

In short, I would expect that you are entitled to a refund of 75% of the Avios, as well as 75% of the carrier surcharge (but not other taxes and fees). However, I am not sure whether there is binding precedent on this.