Then I would be careful calling it a "trip in vain"... because this is not what it is nor would it would cover.

If what I am reading is correct, your REVENUE flight returns out of PEK (presumably a few days later?) to your original destination, and what you'd like to do is use the presumptive (though not yet official) schedule change as a way to change your return to your original destination from SIN the same day as you arrive (instead of PEK days later).

If you continue to use the term "trip in vain", especially when you call United to discuss this, I assume UA might question why you would take the trip at all and you're not going to get anywhere. Without knowing all the fare rules, I think it's safe to assume you can always ask, but in this case it seems highly unlikely. The best you might get is the offer to simply cancel the whole ticket without penalty and get a refund. Personally, I am not sure how a schedule change into your destination city would affect your return out of another city, and United's published rules for schedule changes take this into account:

"If your scheduled departure or arrival time changes by 30 minutes or more, we're happy to try to find other available flight options that meet your needs. Please keep the following in mind when you call:

• Your origin and destination will have to be the same as on your original itinerary."

Also, "When a schedule change happens, we try our best to provide you with options that minimize the disruption to your travel plans. In cases where the new flight options don't work for you and one of the following scenarios applies, we may be able to offer you a refund:

• The scheduled departure or arrival time changes by two hours or more.
• The change causes issues with the overall length of the trip, such as making the connection time too short or significantly longer than it originally was."

Given your circumstance, bullet pt #2 above does NOT APPLY (since your "onward" ticket is separate award ticket/trip), so unless the SFO-SIN change is more than two hours (bullet pt #1 ), I'm afraid you're out of luck.