Hi,

Just did US938 DFW-CLT-LGW through-flight. I have asked for the same (to get more miles), the check-in agent was very nice, but he said nothing can be done as then the e-ticket would desynchronize (as e-tkt shows DFW-LGA) and boarding passes would not print. He would have to reissue my ticket, but this would require to pay change fee plus upgrade to the lowest available fare (I think to B-class).