score:2
I think I managed to avoid the change fees. Here's what I did:
I split the existing (round-trip for both passengers) itinerary. Now, (1) has their own itinerary (and confirmation number, etc.) for the A → B → A trip.
I cancelled the round-trip flight for (1). This meant that, as of this stage, (2) is flying on the round-trip flight alone and (1) is flying nowhere. This accrued "future flight credit" equivalent to the original expenditure on (1)'s ticket. United offered the information below about change fees.
I booked a new one-way flight for (1) from B → A on the same flight that (1) and (2) were originally flying on. (This will be the last leg of (1)'s multi-city trip.)
I booked flights for (1) from A → X → B, the first legs of (1)'s multi-city trip.
This resulted in three itineraries:
As far as I know, this resulted in $0 in change fees. With that in mind, I have yet to use the "future flight credit" on another flight, and I may encounter fees at that stage.
Upvote:3
The good news: You can consider each person separate, even if you bought the tickets together.
The bad news: if a person doesn't fly any leg of the ticket, the rest is typically considered void and lost. So if (1) doesn't fly his first leg, his complete ticket is gone. Your only option option is to work with United, and change that ticket to fit the new plans (and pay the fee).
Next time, when you pick an airline, consider their fee structure and your change risks, and potentially pick another airline with less or no change fees.
Note that your argument (buy even another ticket) doesn't matter to them - change fees are not covering the cost of the change, that is zero or near zero, it is simply a revenue-generation mechanism that lives from people's plans changing. They take it because they can, not because they need to cover expenses.