You have to choose one discount or the other. You can't get both FD and the
AAA discount.
As for downgrading to the QSDP, I know in the past they've offered people on the regular dining plan the choice of trading in a TS credit for a CS credit plus something else during FD. The reason being people couldn't get ADRs. I think even outside of FD, you could trade in the TS credits, but I'm not sure how it was done.