I believe that Jesus, Moses and Elijah transfigured on the mount not in heaven.  When Jesus was praying he became as a bright light and Moses and Elijah appeared with Him.  Jesus had brought James, John and Peter up on the mountain and as such, were witnesses to this event, although Matthew, Mark and Luke wrote of the event.  Matthew 17:1-8; Mark 9:2-8; Luke 9:28-36).