Semele was the mortal mother of Zeus' son Dionysus. She was tricked by the jealous Hera and she ends up dying.
Dionysus survived, but when he grew older, he was concerned for his mother and retrieved her from the Underworld:
Dionysus feared for his mother, whom he had not seen since birth. He bypassed the god of death, known as Thanatos, thus successfully returning Semele to Mount Olympus. Out of the twelve Olympians, he was of the few that could restore the deceased from the underworld back to life. 
However, once Semele was among the gods, Dionysus refers to her under the name Thyone:
THYO′NE (Thuônê), the name of Semele, under which Dionysus fetched her from Hades, and introduced her among the immortals.
Was there a specific reason for this name change?