Not true. Same day confirmed changes can be made for later flights. The problem is that the OLCI engine does not allow for this; it only ever shows earlier flights.

Although a pain in the butt, I've been able to get around this by going to the airport first thing in the morning, paying for the confirmed change, attending my meeting, and then taking my new flight later in the day.

Whether this will work or not is a guess. I've only ever done this on domestic flights; I don't know if this would work for an international fare or what they might try to charge in this case. SDC is also subject to availability; if the later flight is oversold this plan definetly won't work.