Highly unlikely to get downgraded in Biz on SQ. One of the safest types of booking as SQ is not aggressive in overselling (and don't oversell F).

It may only happen in rare operational cases where an aircraft goes tech or so and the flight gets cancelled and pax get reaccommodated onto next flight(s) and there may not be enough free seats to go around. In such cases just insist on the earliest option in booked class of travel and don't accept a downgrade. It'll be up to SQ to make it happen.