Assuming your booking class is I (It is change fee + difference in fare!)
for tomorrow:
MCO-PHL-LHR
MCO-CLT-LHR
or MCO-DFW-LHR
For today there is no I class available.
You could suggest a downgrade to WT maybe. There is availability for that.
In a similar case BA graciously waived the change fee for me, but that is not a right you have.