I was assured by a UA agent that my full balance had to be used on my new itinerary (S class international, wanted to change the departure of a $1000 ticket so I would have an $800 credit once change fees were taken out). She went so far as to say that I couldn't buy a $750 ticket with my $800 credit, even if I was willing to throw away the $50! Does anyone know if their is any validity to this or was the agent just "mistaken".