Just did it a few weeks ago. Was scheduled on BA from AMS to LHR (codeshare AA), then AA to DFW and beyond. BA was cancelled (weather), so we were rerouted on KLM and United. Called AA Customer Service and since it was all AA codeshare originally, they gave me full credit for the original trip. Two of my colleagues did the same. AA was great about it in every case.

Only bad part was BA not letting me into the lounge since I now had a KLM boarding pass., but that's another item.