Well, the food isnt exactly the same. However its not vastly different.
The F seat is similar to the 77W J seat, but a bit tighter and the cushioning is definitely cheaper and not very comfortable. There was one instance I was in F, and J was quite empty so asked if I could take a nap in an empty J seat as I find that more comfortable for sleeping.
But the above notes are correct--AA has difficulties managing if on BA ticket and even if on my BA FF number instead of my AA lifetime sapphire number--it just really screws things up for them.
You of course can ask, but I'd expect the response to be a full ticket exchange which day of travel will be quite high, so in advance I'd give them a call to see what the current cost would be to upgrade with a new ticket because yes, AA do not upgrade to F on the transcons. I think a few years back they may have even stopped allowing us to use a system-wide upgrade from J to F (similar to gold upgrade certs). But its been a few years since I've been an AA emerald, so just dont try anymore!