Detective Conan OVA 9: The Stranger of 10 Years

Synopsis:

While lying in bed with a cold, Conan receives a phone call from Haibara offering him a chance to try the latest prototype antidote to the Apotoxin-4869 that turned him into a child. Thrilled at the chance to turn back into Shinichi Kudo once more and surprise his friends at school, Conan immediately takes her up on the offer, despite his high fever. After his transformation, Conan is amazed at how different the world looks; but after arriving at school he slowly begins to realize that he’s somehow managed to travel forward ten years in time!