myitins ==> view/modify ==> change itinerary ==> check the price of the itinerary

It tends not to work when it's a multi-city booking, or, sometimes, after a schedule change or other change to the itinerary. Again, online refaring is not recommended when you have confirmed upgrades with no confirmable space left on the flight, or, maybe, when you have waitlisted upgrades (you may lose your place in the queue).