From the Comprehensive Rules:
711.12. When a double-faced permanent transforms, it doesn’t become a new object. Any effects that applied to that permanent will continue to apply to it after it transforms. Example: An effect gives Village Ironsmith (the front face of a double-faced card) +2/+2 until end of turn and then Village Ironsmith transforms into Ironfang. Ironfang will continue to get +2/+2 until end of turn.