The OP is not flying FRA-LHR. He is flying SFO-XXX-LHR-FRA on the outbound, and -- apparently -- FRA-CLT-SFO on the return.

He might or might not be allowed to drop the LHR-FRA segment on his outbound award; could depend on married-segment/divorced-segment issues. He could cancel the outbound award, and rebook SFO-XXX-LHR, but that might now cost more miles than his original booking.

It's not clear whether the outbound and return are both on a single ticket, or if they were booked as separate tickets.