Vayeishev

In the first dream he says they were all binding stalks of wheat and then his stalk stands up and their stalks bow to his stalk.  In the second dream the sun, moon, and the stars (that represent his brothers) are bowing to him, himself.

When they go to Mitzrayim and meet Yosef who is in disguise (42:6) it says they bow to him. But they are bowing to him, as he was just described, because he was the wealthy ruler of the land.  Thus, in the first dream they bow to his stalk, his possessions, the wheat that he controls.  But the second time they see him they bow to him himself (In Perek 43), not just to what he provides.

- Mei'Otzreinu HaYashan Pg. 196. citing Knaf Ra'ananim