I am booked on 919/1565 LHR-IAD-MCO on 30th April. I booked through a travel agent.

I want to see if it's possible to move to 923/1561 instead as this arrives at MCO 3 1/2 hrs earlier. When I priced both on ua.co.uk the 923/1561 comes out cheaper, so in theory should my agent be able to book this, or can i ring reservations and see if they can move me onto 923/1561 ?

Are you ticketed yet? IF not, it should be easy. IF you are ticketed, it's more complicated. You can't change to a lower fare without a change fee. But if you can find a fare that it higher than your current one, you likely will be able to do it without a fee at all.

I'm not bothered about paying a lower fare at all, I just want to be on the earlier flight. I was just pointing out that the flight i want to change to was cheaper on ua.co.uk, even though i didn't book through UA. I assume I am ticketed - i paid the full balance off at the end of january?

Based on your posts, I'm assuming you're flying on a discount fare (i.e. not a Y or B-class full fare ticket). If that is the case, then whether you can make a change or not depends on the restrictions with your particular far. Most likely, you will have to pay a change fee even if the new fare would be lower, because changing flights requires you to be reticketed and thus the change fee.

I would suggest calling your travel agent first to see if they can do anything for you, since they were the ones who issued the ticket. It wouldn't hurt to call United reservations, too -- they should be able to tell you what your options are. I wouldn't be too optimistic, though; you'll probably end up having to pay the change fee. If you do, you might want to check your return flights at the same time, as you'll be one change fee per ticket per instance, regardless of whether you change one direction or both.

You might also be able to try to go standby at the airport, although I've heard mixed messages as to whether that's possible for international flights.

As others said, if the fare is lower, then you need to pay the $100 change fee. If the fare is higher though, then you only pay the difference. I would wager that there is a more expensive fare that is less than $100 more, at least one way.

I would go to the Travelocity fare listings page and see where your fare is. Plug in LHR-MCO on UA and then look at the next one up, make sure you meet the fare rules, then subtract what you paid (without taxes) from the new fare. Divide that in half (since you only have to upfare one way) and that is what you need to pay. The bonus is that you can change your return dates if there is sufficient availability on your desired flights.

Also note that 923 is a 767, where as 919 is a 777.

Correct me if I'm wrong, qasr, but I thought that for most non-refundable fares, if you changed flights you would have to pay the change fee in addition to the fare difference. I thought it was only if you upfared on the same international flights that you could avoid the change fee.

Also, AdamUK, as qasr implied, you don't get any credit for the fare difference if you make a change to a lower fare. In other words, they won't offset fare difference against the change fee.

You can use the full value of most international restricted fare tickets toward the pruchase of a higher fare without any change fee. So if the new fare is $3 higher, you just pay the $3 differce.

Keep in mind that not all change fees are $100 anymore. Many international tickets have a $200 change fee.

This is new to me. I have changed flights many times before with no fee, but I was just charged $200 for changing to an earlier flight from FRA-IAD, no waiver for 1K whatsoever. Maybe there is a distinct difference between same-day confirmed change versus stand-by.