just buy another ticket and walk away from the LAX-IAD leg if you don't have additional downline segments that you need on the same PNR

another option is to try to look into SDC'ing to LAX-DFW-IAD once you get to LAX. probably will not work, but it's one way to get UA miles out of the deal and no more $$. i wouldn't want to rely on it as my sole strategy, that's for sure. but in conjunction with an anytime fare on WN it might be an option.