When I started playing DnD, we treated leaving a threatened square as actually leaving the entire threat range. So moving from one threatened square to another on the same creature was never a problem.
Now recently my friend has been playing and he's a tad more versed in playing and says doing so incurs an AoO. I'm inclined to believe him since he's been doing this for years but I felt I should check for every ones sake at the table. Is he right?