In the seven or so international refares I've done involving confirmed upgrades, the scenario described above has always been the case. I've never had a problem doing this but I all I am changing is the fare class or just the fare paid. I will add that all of these involved the use of SWU's rather than miles, although I do not think that is a variable other than maintining an H fare or higher for int'l mileage upgrades of course.