If I purchased a ticket that booked directly into "I" or "A" or "P" class, and then got involuntarily downgraded, and AA refused to refund me the difference to the lowest available coach fare on that flight, you can bet there would be a lawsuit. I might even make it a class action, because I'm sure they've tried to screw other people this way.
Think of it this way: I can buy a "Q" fare and confirm a seat in economy for $200 (one-way), or I can buy an "I" fare and confirm a seat in business class for $1000. I choose the $1000 "I" fare. If something happens and I end up flying in coach, I will be arguing for an $800 refund. What jury will refuse? The argument is pretty straightforward in the absence of any explicit language in the fare rules or contract of carriage indicating otherwise.
I'd point out that there are $2,200 one-way -UP fares on JFK-SFO that book into D inventory (fare basis YA0UPDMR). If you think AA can get away with providing coach service at that fare with no fare differential refund, you're sorely mistaken.