As Rochel was dying in child birth she called her newborn son"Ben Oni"the son of my pain and mourning. Yaakov called him BinYamin the son of my strength from the word "Aven". Why would Rochel give such a depressing name and how could Yaakov deny her dying wish?
The Chasam Sofer explains that Rochel's use of the word Ben "Oni" referred to the child of her strength. How? The reason Rochel died when they came to Eretz Yisroel is that Yaakov and all the Avos kept all the mitzvos while in Eretz Yisroel. Since he couldn't be married to two sisters, Rochel being the second sister he married was doomed to death upon their return from Chutz La'Aretz.
This all happened to Rochel because of the strength and courage that she showed in helping her sister Leah marry Yaakov on what was to be her wedding night. Her death was another tragic consequence of this heroic and selfless act. Therefore she called BinYamin the son of her strength.
Yaakov didn't change his name, he merely clarified it since Ben Oni can have the negative connotation of pain and mourning. He therefore called him BinYamin to let everyone know Rochel's true intentions upon her death