Just to update this, this actually did happen to me today. The rep insisted that since my fare was a Saver, she could not upgrade me. I insisted (politely) that that just wasn't the case and I had done it before. She put me on hold to talk to a supervisor, and when she returned, she had already upgraded me. No, "I'm sorry I was mistaken" or "huh, turns out I can upgrade you."
In her zeal to do the task while I was on hold and and without acknowledging what had happened, she applied a coupon I have that is good through November and have planned for a trip later this year, rather than a coupon someone here kindly gave me which expires this month. She then had to redo the whole thing when I gave her the expiring coupon number. Frustrating, but it ended OK.