In these situations, I try the app or web first before calling an agent. Sometimes it seems like the "Change" function is different from the "same-day change" function and they always seem to try the prior and have you pay fare difference. Also, I've had many times where I see inventory on the UA site (or a specific price) and then after calling in they claim the inventory doesnt exist (and the price is much higher). They will say that their system is more accurate than the website. Sometimes I can go ahead and use the website or app and it works eventhough the agent cant. Sometimes pushing them to call the rate desk resolves it but then you're often on the phone for an hour. And sometimes they're right, it errors out when I try it on the app/website. No idea why.