I had a quick look at this link, https://www.britishairways.com/en-gb...and-conditions and it appears pretty vague and seems to point to the conditions will be stated at "booking". It also references calling which you have done and got mixed results (no surprise there). Hopefully others will reply with more first hand experiences in making changes and how it really works. Personally, I would keep calling until I got an answer I liked since the answer seems to change so much.