yes, they can "insist" that you use the entire ticket and the baggage must go the whole way. But, I can tell you that as a gate agent, I can and HAVE waived this rule if the request seems logical.

last example i can recall...pax had a 3-hour layover and the weather was not looking good. Asked if she could get off in XXX and have someone drive her to her final destination (she could have easily gotten there in the time she would have been sitting on her layover). I made sure she was on the final portion of her ticket (she was), deleted the 2nd segment, and checked the bag and the pax to the original connecting city.

I'd wager that you wont find many agents who will do this, but I reason:

1. We already have her money.
2. I essentially "freed up" her seat in case someone wants to buy it full-fare at the last minute.
3. Happy customer=repeat customer (no, really?)

Don't risk getting lucky at the airport. But if you call reservations and tell them your plan, they might deny you over the phone AND make a notation in your PNR. Then you're really hosed. I'd suggest pricing out a new ticket, and then applying for a credit and putting that towards the new ticket. good luck.

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"A mile of highway will take you a mile,
but a mile of runway will take you anywhere."