This also happend with a DFW to LHR that goes via ORD. Leaves as an MD-80 out of DFW and then as a 777 to LHR out of ORD. But I do recall that on AA.com is sais under the details of the flight that there is a change of plane.

We use to to this flight with a return via EWR from LHR to DFW (that flight is no more). It carried one flight number BUT the 'system' use to give you two flight segments and two miles, one LHR to EWR and EWR to DFW. Never could figur out why but may for going thur customs/imigrations??