"dk" is thinner...it knits at about 22st=4" in st st
"worsted" is thicker...it knits at about 18-20st=4" in st st
Many cardigan and pullover patterns ask for worsted weight, or even thicker. When you find a pattern that you like, you must hunt for the right yarn, starting with the thickness of the yarn.
You could not make a dk yarn behave like a worsted yarn. If you forced it, like going up in needle size til you achieve 18-20st=4", the resulting "fabric" would prolly be very mesh-like. Too loose.
On the other hand, you couldn't make a worsted yarn behave like a dk. If you went down in needle size, forcing the worsted to knit up at 22st=4", the resulting "fabric" would be thick like a rug, or mat!
So I was excited when I saw that EL has produced a worsted weight Silky Wool. It is such a nice blend! Really soft, but doesn't pill. Displays cables very crisply. Also produces an interesting stocking stitch as well! Also, rev st st!