Booking an Award from LHR Airport

FTers,

My Question:
Can I change an award ticket from LHR's United counter?

Background:
I am planning a trip and am trying to book an award in biz the whole way. The last segment of the trip (CDG - PHL) I am currently in Y. I've been watching when availability releases, and it is predictable. If there are at least 12 seats still available in F when I arrive in London there will be 2 award tickets available. I just need to figure out how to change my ticket.

Thanks Again FT!
marben