When peple refer to the weight of a yarn, they are usually saying whether the yarn is DK, Worsted, bulky or chunky and that refers to the yarn gauge not the weight in ounces.
If you want to substitute one yarn for another in a pattern the 2 most important things are gauge (how many stitiches to an inch will you get with the yarn) and yardage (is there enough yards in each skein for you to finish the sweater.) This information is on the label of the yarn you want to use.
The instructions for the sweater will list the recommended yarn and it's gauge and yardage. You just have to make sure that the yarn you want to substitute matches the gauge and that you buy enough of it to get the yardage you need to finish the sweater.
The oz weight of the yarn is not really important.
Does that make sense?