Just an update to this situation - tried chat initially and after horrendous wait times, got connected to an agent who said they can only do updates to dates. I asked if I can speak to a supervisor to request for an exception - they said they can arrange a callback which they did.
About 2-3 days later, I get a call from SG number, I explain the situation about the connecting flight and they suggested they want to try and help me would put in a request for destination change (and date change) and once they receive approval within 72 hours, would call me to confirm. After about 8 minutes, they called me back to say they already received the approval to proceed and in no time, they were able to update the booking. So now an overnight layover in SIN vs overnight layover in SFO + extra connecting flight. Agree to most comments on this forum about how agents based out of SIN are usually the most helpful.
I expressed great thanks to the agent. I had also asked if it was possible to change starting airport to avoid layover, they did check but said they're unable to do so because - different country. So I believe the same geographic region change is something the agents in SIN can consider on a case by case basis.