Hi, my situation is a bit complicated, not sure if there's anyone who can help...

I have two reservations made:
1. Delta - MSP > JFK on Day 1
2. American - JFK > LAX on Day 2 (Business - R Class, Reservation made with Cathay Pacific as I have a subsequent flight connecting to Hong Kong.)

I want to spend more time in MSP, and therefore, I am considering changing my AA booking by getting onto a later flight on the same day from JFK to LAX (and if successful, I can change my Delta booking afterwards).

However, Cathay told me that R class has all been booked for all later flights on the day of my travel. I asked if they could upsell me, but the answer was negative due to some ticketing restrictions.

Therefore, I am now thinking if I can

1. get to MSP on Day 1 and visit the AA check in desks (within 24 hours before my departure time), and see if they can move me to a later flight? Do they have authority to make changes given that I am not departing from MSP but JFK?
2. make changes to my booking online (AA website) / via AA app 24 hours before departure - the problem is I was ticketed by Cathay, so I'm not too sure...
3. contact AA right now to see if they can bypass Cathay to get me another flight?

And may I also ask, if R class is still not available for other flights 24 hours before departure - can I request for an upsell to other fare buckets / Flagship First, such that I can get to another flight? If so, do I do it online or at the check in desks?

I hold Cathay Pacific Diamond status (= oneworld Emerald), not sure if that matters...

Sorry for the long and complicated question, and thanks in advance.