I have 2 saver awards booked in F: LAX-ORD-LHR.

On the return, there is a direct flight from LHR to LAX leaving within 45 minutes of my LHR-ORD flight.

Assuming that there are (only) revenue seats available in F, is there any chance a request to switch to the direct flight would be granted when I check in for my flight? Would I have to pay a change fee?

Thanks