Originally Posted by
jsloan
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OP wants to purchase a ticket, LAX-DEN-YYY, that will serve as an EXTRASEAT from LAX to DEN, but will be a regular ticket from DEN to YYY.
(In addition, OP will have a lap infant, which is going to complicate things even more).
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If that's the case, then you're right, no it will not work.