You will have lounge access at Changi, as long as it's an SQ flight number, codeshare with Scoot notwithstanding.

Do note however that Scoot doesn't have Premium Economy, it's either Economy or ScootBiz (essentially a better seat with a complimentary meal, rather comparable to most full service airlines' premium economy). Believe if your SIN inbound flight is in C on the same PNR, your onward class from SIN should be ScootBiz.