Sorry if this has already been answered recently - did do a search against 'buy up' and could not find anything

We booked international flights to the uk and had to reschedule yesterday, to move flights back by a week.

First csr told me that because we were 'buying up', we would only pay the fare difference and would not have to pay an additional $200 per ticket change fee.

When I called later to actually make the change, the csr told me that the original csr had made a mistake and that we would also have to pay the additional $200 per ticket change fee - she said that the original csr had not checked the original fare basis correctly

What is the scoop here ?

(In the end UA csr was great and agreed not to charge the additional $200 per ticket, because we had been given 'wrong' info by the first csr - however, I would still like to understand the rules around buy-up's)