Go to SCL before any BA flight departs and find some BA people. Tell them the call center clowns are total idiots in regards to this change and are disregarding the "use historical fares" fare rule.

eta: Also go ahead with that complaint. I have nothing but positives to say about the BA on-shore agents (nowadays staffing the GGL UK line during UK daytime only) and nothing but negatives about the off-shore agents. Your complaint is not going to change the world but it's similar to how voting in the election works. Do your part.