I think one more clarification - the flight credit for OP when canceling can't be used to purchase the girlfriend's ticket, OP would would have to use the credit on his own for a different flight before the expiration. Unofficially there is rumor that you can call Southwest and have an expired credit issued as a voucher for a fee, which might be $100 but subject to change. If OP is in it for $400 and won't be able to use the credit, then it may be worth trying to do this, although it isn't guaranteed to get $300+ of vouchers out of the deal. However in each case you'll still have to pay cash or points for girlfriend's ticket before she can add you as companion for the flight. Make sure she has you named as designated companion in her RR account also before canceling anything.
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