Depends on a few things. The most important thing is the gauge. So if the pattern you have gives a gauge of 5 sts per inch, then if you get that gauge, it will come out right. But let's say you're using an aran yarn, which was really intended to give 3.5 sts per inch-- trying to make that thicker wool pack into 5 sts per inch with much smaller needles will make the size come out right, but you will probably have a very stiff sweater. There are ways to convert it though. If the construction is very simple, then I'd go for it. But if the sleeves are set in, or it has a complicated collar, I personally would just go buy some DK wool, or find a pattern that would be more suited to my yarn (try www.knittingpatterncentral.com
for free patterns). But if you want to convert, just make sure the gauge will translate. Meaning, let's say the original pattern has a gauge of 5 sts per inch and they tell you to cast on 60 sts for the back, which would mean a 12" back. And let's say your aran wool gets 3.5 sts per inch. Multiply the 3.5 times 12 and you get 42, and so you would cast on 42 sts instead of 60, to still get the 12" back. Make sense?