It's a GA thing, not an FA thing. They can't bring you up front unless the GA hands them new BPs. If I was in that situation I guess I would have positioned it to the GA like this:
"Hi, I'm on a paid F ticket and I was rebooked on this flight for a mechanical. I see you are processing upgrades. I would appreciate if you could accommodated me in the class I paid for." Even on a deceptive A/P fare I think that has a pretty good chance of working.
My money is that mbarreto was on the A or P fare as a month doesn't go by that someone doesn't post about getting screwed by them here in FP (I shudder to think of how many people this actually hits each year). The CoC does not specially address the remedy for A/P downgrade. So if that's the case the OP is entirely at the mercy of however DL wants to interpret the CoC at that moment. Your Millage May Vary.
Next time one of you are at the DO please point out that at the very least the remedy for cabin downgrade should be spelled out in the CoC. I don't think that's asking too much.