Been to korea 7 times. Always change there cause rates there better. Insadong, Myeongdong, Dongdaemun, Seoul station all have. Walk around see which one got best rates. Bring $50 notes, most money changers don't accept bigger denominations like $100 or $1000 SGD. Personally I feel the rates at Insadong has always been the best. Last few trips always go to same money changer at Insadong, near Jongno-sam-ga station.
For those thinking go korean bank change, 100% must bring passport with you. No passport = no change.
Change abit in sg for transport & some spare for food etc.