This exactly happened to me in July.
I suggest contacting someone on the phone as by email they are utterly useless.
My experience: 6 emails to get back what I paid additionally in business (versus economy). Then ignored all emails about differences in tax. Eventually had to phone up. No such thing as compensation offered. Terrible people to deal with - just using up remaining avios and switching elsewhere. Still haven't been paid back extra taxes owed. Pathetic.