Here's what I would do:

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If that doesn't work (and it should) then do:

2. LAX-SFO isn't cheap. It usually runs about $170 roundtrip (sale price) on UA. So, instead of paying an additional $100 to change and pay the difference of one way (stupid). I would cancel the entire reservation and take $170-$100=$70 as credit. Buy a ticket on WN and fly LAX-OAK or B6 LGB-OAK (I know, I know, but its cheap!) for only $120 one way with no restrictions. Use your $70 UA credit next time. That way, you don't have to pay the difference in fare and you get to keep the credit.

Good luck!