I had a BE ticket on hold and called the agent to book using a Flight Credit today. The agent tried at length to up-sell me to a Main ticket which I declined, but then once I gave her the Flight Credit number I was told the same, that since the original flight wasn't BE it couldn't be used. I reached out to the AA Twitter team and they confirmed this because "... BE tickets are instant purchase deals and the cabin fare can't be downgraded in the ticketing world". It seems the T&Cs are misleading!