I wouldn't worry, though it depends a bit on your final destination. Usually the App witll refresh (not MMB by the way) and give you another USA to UK option, which you do not have to accept. The onward conneciton - I'm assuming it's one booking - may not initially shift, but if it has plenty of flight options then that can be sorted out at JFK / EWR. This thread gives more advice:
https://www.flyertalk.com/forum/brit...need-know.html