I've had UA gate agents accept other airline's tickets numerous times at no cost to me. And I'm talking about advance purchase, discount fares. Don't even talk to agents on the telephone. They will say no every time or charge you a full Y fare to do so.

Two notes and a caution that may be of interest or help:

* I'm told that it does cost any airline quite a bit to process a ticket on another carrier's ticket stock. Things hang up in the auditing trail.

* It's even more hassle with e-tickets. You basically have to get the issuing airline to generate a paper ticket. Last September 13, almost every airline was generating paper tickets, issued on any fare basis, and endorsing them to any other carrier. But those days of emergency good will are gone.

* Be very careful if you get another carrier to lift the outbound segment of your itinerary. That will basically constitute a no-show with the carrier for which you were originally ticketed. The rest of your reservation will usually be cancelled. Since you can't count on friendly agents being downline to do you favors by taking the return portions of your ticket, you need to be sure that your reservation is preserved somehow if you are planning to use the rest of it.