When I do on a random route a mixed (economy out Business back) it does show £50 change fee and you are right it doesn't say if its different for CE. However if I do that same route as CE on all legs then it comes up as £100 for the change fee. So my guess is that BA website is just wrong and not able to show that the economy sector is £50 and the CE is £100. So BA management my booking correctly charged the fees. I do have an issue to mis-sell it at the first place so if you feel strong about it, I would take some screenshots and ask for a refund of £50. I'm not sure how far you want to go and is your time worth the £50, that's up to you.