Following up, mainly for anyone who may come across this in the future....

I called JL, they were very nice. They couldn't upgrade the ticket because it was issued by a travel agent, which I expected.

Then he said what Dave Noble posted above: I would need to call the travel agent and upgrade the entire journey to First Class, which I'm not doing. Since there are only 2 seats available in first, I expect it to be sold out by the end of next week. I will check at the check-in counter if a cash upgrade is available for AA on their plane (probably not, but maybe.)