Welcome To FlyerTalk.
Just call in and state you have already book and paid (non-refund) a local tour at SFO for your initial 12 hours layover, hence this will not work. But do first research what route are available before you call, so you can present that option to the phone agent right now.
Also the YVR-HKG flight will be on the new 777HD, so if you are one of the people that do not like the new config, this can be another reason for your complain as well.
I checked the available flights and it looks like nothing routes through San Fran any more. Only have stopovers available in PVG or PEK.
My original flight plan was
UA378 - YVR -> SFO - 319
SQ3 - SFO -> SIN - 77W
If I were to hazard a guess, it'd be the SQ3 flight getting changed? I have no clue how to check it though.
My new flight plan is
AC7 - YVR -> HKG - 77W
UA895 - HKG -> SIN - 777