Well think about it for a second. If Conan returns to his normal body the main story line will end! What might happen is like most long standing animes, they will either let it died leaving the fans wanting more or will start a new series with a whole new back story much like Naruto becoming Naruto Shippuden. Does that make since?
Considering That Conan is Shinichi and that he's human,he grows each day..but I do recall there being an OVA "Stranger in 10 years" when Conan grows up.
