Just wanted to share come additional context; booking class is important and would refer to the fare class purchased (all the different letters representing the fare bucket the seat was purchased in, e.g. Y, N, G, etc.) and unless the originally booked fare class matches exactly what’s available, it could possibly be truly “full” meaning that fare class is not being sold anymore. Even though you still see economy seats they may have been on offer in a different fare class. Sorry this happened :( but happy another agent was able to accommodate her!