You have to check near towards your departure. My SQ161 was scheduled to be 3-class when I booked it in early Feb. But by the end of Feb, I was notified as 2-class service only using A345 Leadership. So I changed it to different schedule with 3-class service.
My flight SQ164 20 Sep SIN-CGK was booked for 3-class service in Jan. And I got the notice last week for changes in booking to J from F, or else take other flight with 3-class service.