If my experience is anything to go by, maybe not. Although some tickets did open up from IAD though by this stage we were already on our way up to Toronto. So maybe JFK was just particularly bad and you will have more luck from ATL or DFW.

As we literally waited til last minute hoping a flight from JFK would open up we couldn't make payment before we jumped on the bus. No problem, we were told twice, just pay at the airport, any time up until check-in closes. Our bus arrives late and we reach the airport 2.5 hours before the flight rather than even earlier. No wi-fi to call but we emailed just to be safe. Our seats had auto-cancelled 10 minutes prior to us reaching the ticketing counter and could not be recovered. No guarantee we could get new ones (though who could possibly purchase that flight at last minute?). So we had to wait until everyone else had checked-in before we could be issued a ticket. Phew.

Put nicely, KE are rule-sticklers. From a management perspective, I would say there is a serious case of not giving lower level management sufficient authority to make common sense decisions. Though, as stated above they are certainly not alone in the aviation industry.

FWIW F was basically empty, as I expect you will find too. However, due to said reasons, you might want to keep your expectations of a seat opening up real low.