Changing after you fly the outbound locks in the inventory which is not necessarily the same as the price (the outbound will reprice if your new travel does not satisfy the fare rules of the outbound fare component).

As a general rule, if your current inventory class is still available on the outbound it doesn't matter. If it is no longer available, it is to your advantage to wait until you fly it. The standard templated UA international change rules permit using historical fares and current inventory.