UA can't revalidate it because it's SQ stock. SQ can't revalidate it as they don't hold the live segments and it's a UA flight. (IATA and/or Star *really* need to get their act together to allow interline reval of ETKTS).

The normal procedure would be a even exchange reissue with no fee to get the correct flight/date on the ticket - it's a free "reissue" as it is in effect a reval.
Some carriers seem happy to input a message to the check-in at that segment to accept the current coupon for travel - same routing, just a different flight/date so they still get paid for it.

If all else fails and you have started the journey, please direct the agent or the airline to the changes paragraph in the rules.
"AFTER DEPARTURE
FOR ALL REMAINING SECTORS CHANGES TO CARRIERS / FLIGHTS /
DATES LEADING TO REBOOKING/REVALIDATION PERMITTED AT NO
CHARGE - ALSO IN CASE OF E-TICKET

- CHANGES IN TICKETED POINT / FARE BASIS / FARE AMOUNT
LEADING TO REROUTING/REISSUE PERMITTED AT A CHARGE OF
USD 125 (CHILD/INFANT USD 125) PER REISSUE"

You're not doing any of the second paragraph so it should be the first paragraph that applies, especially the part about no charge - also in case of E-Ticket. There may be some sort of local service fee/charge, but certainly no recalculation of fare/tax and no reissue fee.