SDC is a UA function. If you call SQ, you’re going to pay SQ’s change fee. To get SDC, you have to interact with UA.

You can probably do this on the app, keeping in mind that there is a very real chance that you’ll find yourself without a seat. You’ll need inventory in the appropriate partner award bucket (X for economy or I for business / domestic first). A change to another flight on the same calendar date with no change in routing is the safest possible non-016 SDC, but there are still no guarantees. I’d feel better trying this if the flight I was changing from was also wide open. If you try this, and you get a message in your reservation display on the app saying that your ticket is out of sync, then you either need to call SQ, pay the change fee, and have them reissue the ticket, or you need to call UA and ask them to put you back onto your original flight.

Changing this in person at the airport would be safer, but I have to assume the reason you’re trying to do this is to spend an extra hour at your origin, so I doubt that helps you much.

To reiterate: This is not recommended and could backfire, but there is a chance it would be successful in this very limited situation.