Only once (in Tehilim) is Yosef termed "Yehosef." I don't think that this means his name was changed, but rather that the letters Yud and Heh, which are part of G-d's name, were added to Yosef's name to testify about him. What exactly is the testimony? The Talmud seems to point out two possible testimonials regarding Yosef: (1) He knew 70 languages, which is born out by this verse which refers to his knowledge of language (Sotah 36b). 2) He maintained his purity and sanctified G-d's name, and therefore merited to bear G-d's name in his name (Sotah 10b).

The Bnei Yissachar adds an interesting explanation to the first interpretation. The verse before this one refers to Rosh Hashanah, and our Sages teach that Yosef left prison on Rosh Hashanah. He received the gift of language with the extra letter Heh (which, in Kabalah, refers to the sphere of sovereignty, Malchus, corresponding to his being conferred with sovereign powers by Pharaoh). This is also the main aspect of Rosh Hashanah, when we proclaim Hash-m's sovereignty in the world.

Yoel Domb