Before you purchased the one way ticket PDX-LAX did you call UA and find out what the cost would be to change your flight with them to arrive at LAX a day earlier? Since you would be changing part of your return itinerary and not your outbound it would not require them to cancel the whole thing and reticket it so you'd just be on the hook for the change fee as long as the same booking code was available.. was your PDX-LAX one way that cheap?