A head change is not simply the point in which you notice you get high. A head change can only be felt by a sworn toker who smokes every day and has been smoking so long they really don't get high anymore. A head change can be achieved by simply exhaling most of the oxygen in your longs and taking a huge bong rip. The lack of oxygen and presence of marijuana smoke gives you a type of head rush in which a vast majority of experiences can ensue. It is sometimes unpleasant and shaking can occur. A true smoker enjoys this head change and will look for it at the end of every hit.
Bong : "Bubble bubble bubble bubble bubble bubble bubble, shuuuuuu, bubble bubble bubble."
Joe: Whoa that was a huge hit you got greedy you bastard.
Derek: oh dude I just got the craziest head change
Joe: I know I saw you shaking I'm glad I took the bong from you.