"If I ask at AA flagship check-in will they be able (subject, of course, to availability) to move me onto this flight"
What flight is
this flight? If you mean your originally-ticketed JFK-SFO flight that was the subject of the schedule change, AA permits free same-day standby for oneworld elites (all levels) on domestic flights; $75 fee for non-status flyers. Do you have oneworld status through any airline?
AA policy here:
https://www.aa.com/i18n/plan-travel/...day-travel.jsp