A Rediculous Assignment

Voldemort is dead at the end of Harrys fifth year, but something seems off about the boy so Albus sends Severus Snape on an assignment. What does he have to do? only deage himself back to 16 and start his sixth year over again as a gryffindor to figure out whats going on with Harry. He accepts, but he doesnt expect his entire life to change along the way.