MyAnimeList has published a database entry for Detective Conan: Episode One - Chiisaku Natta Meitantei.
In this entry, the synopsis reads:
The "real" episode 1 of Detective Conan written by the Mangaka Gosho Aoyama.
What makes this episode the "real" first episode? Was the story in the original first episode not by the mangaka?
That is a two-hour remake of the original 1st episode, in celebration of the anime celebrating 20 years in broadcast. The special retells what happened on the day Shinichi’s was transformed into the body of a child. The special includes new scenes that weren't in the original manga or the first anime episode.
It is because in first episode shown in anime they made certain changes from the manga. The new first episode is going to be fully made according to the manga. For example, in the manga Shinichi was viewing the transaction through a camera, but in the anime Shinichi was watching without using a camera.
Also, the original manga creator said he added new extra content which was not shown before but is important for upcoming events in the series
