"He switched his hands because Menasheh was the firstborn, the elder."
The Noam Elimelech writes:
This reasoning seems backward. (It should have said he switched them even though Menasheh was older) But the Torah wishes to teach us derech eretz, proper manners, and behavior. Honor your fellow Jew according to what he deserves. Even though Yaakov could have just as easily switched them around and moved them into the positions he wanted them to be in.. he did not wish to embarrass Menasheh, and therefore he only switched the positions of his hands. Mipeninei Noam Elimelech pg 109/110