i guess it all depends how desperately you want/need to get back. picking up the LIS-EWR and then buying an amtrak ticket EWR-BOS looks like a reasonable option in this case. or, if you have a r/t ticket and have not burned the stopover, build a "stopover" at EWR by date changing the LIS-EWR segment, and then do stand-by for the EWR-BOS flight.