BA will do this. They did it to Miss Tafflyer from a CW booking rebooked onto IB in Eco/Premium. BA refunded a pittance in Avios due to the Avios cost of the changed routing on IB being insignificantly less expensive than the CW routing. BA was firm, responded with their policy and when pushed responded again that they would not change their mind. My response was to go to CEDR which was simple and forced BA to reimburse correctly. I do not know why BA sticks to their policy of ignoring valid law, especially as it is now widely known. This was pre-Covid, so maybe they'd handle it differently today.