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.