No need to call again. All phone agents can do is to rebook you according to fare rules. Your only hope is to speak to airport staff (check-in or ticket counter; I'm not sure who would have more flexibility) and preferably to someone up the management ladder. SQ tends to stick to fare rules so the odds are not good, but give it a try and report back here. Most likely you will get the usual "Sorry Sir, but we can't do that" (which SQ is notorious for; no flexibility at all).