Still need the correct fare bucket, unless there's just no flights at all available with the correct fare bucket. Only time I've been allowed to change to flights without fare bucket availability is when there were no options at all in that fare bucket. Schedule change doesn't throw the fare rules out the window. Now sometimes they can force a fare bucket open to allow booking, but that's up to DL's discretion. As always they can fall back on the if you don't like what we give you we can refund the fare.