While it's definitely true that LH is the responsible party here should FRA-LHR be delayed, if you're ok with being rebooked on the next available AA option then forget about trying to deal with LH, just go straight to the AA transit desk or Admirals Club once you arrive at LHR T3 and they will rebook you there (on an AA ticket the AA reps are not going to tell you to go back to LH to get rebooked, or anything like that).