Just a data point / tip: If you have seat assignments on an AA codeshare of a JL flight and call AA to
change them, there seems to be a bug. It will tell the agent the new seat is assigned but when they reload the reservation ... it actually didn't change. Fun!
A crafty agent today wouldn't take no for an answer and decided to un-assign the seat and then assign it and that worked. So do ask the agent to confirm the seat change actually worked and, if not, to try the un-assign/re-assign route. Lesson learned: Know exactly what exactly seats you want (with backups) before you call so you don't have to call again like I did
Final fun fact: I am able to use the Amadeus CheckMyTrip app to view the trip with the JL record locator (which I cannot do on the JL website, like many others). And it shows me new seats!