Normally Starwood/Starriott/Marriott reservation confirmation numbers are eight digits (with no letters) but I sometimes get a particularly low number that's only seven digits, so six is probably possible too, but should be very rare. The low numbers I've seen don't use a leading zero, but maybe this is done in the system the agents see, so you might try asking for the reservation number starting with two zeros and your existing number.

BTW, the Starwood Singapore call center always seemed to be very good.