The only way that things could work the way that you’re suggesting is if all fares were one-way and thus didn’t really have any relationship to one another. Most UA international fares are round-trip.

Note that it’s actually possible for a voluntary change to result in a higher fare for the flight you already flew. For example, if you book a RT fare with a 7-day minimum stay, and then you change your return flight to be after only three days, the fare difference is going to be computed by throwing out the fare you already booked and looking for one without the minimum stay requirement.