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As to OP question - yes, you can get a trip credit for the fare difference by simply calling in and just giving them the PNR # and telling them you saw a fare drop and wish for a trip credit for the price difference. I've done it multiple times on the same trip after several price drops. It's possible that fare could have gone back up by the time you call. That risk, though relatively small if done quickly, applies to canceling old booking and rebooking new one.