Agree - the xfer whether legal or not is just so not worth the hassle. If AC won't let you switch for some reason other than the flight being just plain full, ask about paying some $$ for an upgrade of the fare. Having to go LGA-JFK is hassle enough...EWR-JFK is torture unless you want to arrange for a car service to meet you at EWR but there is still traffic and luggage to contend with.