I think that depends on the basis of the award. In this case the TATL must have been in U since MAN-ORD is two class (i.e., 50k ticket), and so if ORD-SFO is three class then the ticket should be U for that leg (i.e., change from Z to U when the domestic two class becomes domestic three class). However if the PAX was coming from LHR, say, in F on a 62.5k ticket, then it should remain in Z.