i'm planning a trip to hawaii with my sister and cousin. they're in SF and i'm in LA. from lax to hnl it's $450, which has a stop off at sfo. but if you leave from sfo it's more expensive. was wondering if it's possible for them to buy the tickets originating from lax, check in online, "miss" the flight from lax and get on at sfo? or would United cancel the entire ticket?