I personally wouldn't due to the fact that they would charge you a change fee and possibly a fare change(?), assuming you've never done this before, you should be fine and UA won't seek payment from you/your wife.
IF your wife eventually wanted to fly from SFO-PDX, she could try SDC'ing the second leg every day to move it later so that she could take it days later when she needs it. This depends on fare class and availability, but I've SDC'd a LAX-SFO segment successfully for 6 days until I wanted to take it.
Just remember to not check any bags that your wife wants in SFO as they will go all the way to PDX.
Go Cardinals!
#Classof14