This sounds somewhat strange, but I suspect that when you called, you were quoted the fare difference between your ticket and the cheapest FC ticket DL was selling at that time, which happened to be A class, with some unknown underlying coach fare class that should be obvious from the fare code. G A P fares do earn 150% MQMs, but most folks here wouldn't consider them to be FCM. Instead, you bought up to FC and according to policy, DL didn't charge the change fee.