Naruto is an extremely popular shonen series that was created by Masashi Kishimoto. It's divided into two parts: Part I and Part II. During Part I, Kishimoto introduced many concepts that seemed acceptable at the time, but, as the story progressed, these concepts affected the quality of the series.
To combat this and to keep the story interesting, Kishimoto made several retcons. Some of them ruffled fans' feathers, but a handful managed to improve the story. As longtime fans will already know, these are the best Naruto retcons that actually helped to improve the series.
7 The Tailed Beasts
When Shukaku was first mentioned in the story, it was not called a tailed beast. Kurama was the only known tailed beast in the story, and Shukaku was considered to be the ghost of a corrupted monk. As the story continued though, the history behind the tailed beasts completely changed.
After some time, it was revealed that Shukaku was one of the nine-tailed beasts that were created by Hagoromo Otsutsuki, who used his powers to split the chakra of the Ten-Tails. Shukaku can be considered the weakest as it only has one tail, while Kurama is the strongest with nine tails.
6 Itachi's Story
Itachi Uchiha is unquestionably one of the best characters in the series. He was introduced in Part I as a villain who didn't fear any ninja from the Hidden Leaf. During his short visit to the village, Itachi beat up his own brother and utterly crushed his spirit. For the majority of the series, Itachi was considered to be responsible for slaughtering the entire Uchiha clan.
However, after Itachi's death, Kishimoto disclosed the Uchiha's horrendous backstory. The backstory revealed that Itachi was forced by Danzo to choose between his brother and the rest of the clan. Itachi could not bring himself to hurt his little brother, so he decided to kill the entire clan. Once the massacre was over, Itachi took the blame upon himself and left the village. This retcon allowed the fans to understand Itachi's actions better, and, with this, he became one of the most loved characters in the series.
5 White Zetsu
White Zetsu was the other half of the Akatsuki member, Zetsu. Originally, the belief among the fans was that White Zetsu was created with the help of Hashirama's cells, and this did not change until Part II.
It was finally confirmed that White Zetsu was created when Kaguya Otsutsuki put the entire world under Infinite Tsukuyomi. White Zetsu was the victim of the Infinite Tsukuyomi, and it was eventually put inside the Demonic Statue of the Outer Path. This helped the series to cover up some serious plotholes, such as why the Leaf Elders or Danzo didn't know about this experiment.
4 More Mangekyou Users
The Mangekyou Sharingan is the evolved form of the regular Sharingan. Any Uchiha who undergoes a great deal of mental trauma awakens the Mangekyou Sharingan. Each Mangekyou Sharingan is unique, and it grants its users different kinds of powers. Furthermore, having the Mangekyou Sharingan in both eyes allows the user to utilize the Susanoo.
The Mangekyou Sharingan is ridiculously powerful, but it does have its drawbacks. Using the dojutsu for an extended period can lead to blindness. When Itachi met Sasuke, he told him that only three people had ever managed to awaken the Mangekyou Sharingan. This was changed towards the end of the series when it was revealed that Shisui and Obito were also Mangekyou Sharingan users.
3 Byakugan Being Better
Part I showcased two of the Three Great Dojutsu, namely, Sharingan and Byakugan. Both of these dojutsu are incredibly powerful, and they give their users a significant edge in battle. The two dojutsu are Kekkei Genkai, which belong to the Uchiha and Hyuuga clans. A non-Uchiha or a non-Hyuuga cannot awaken the dojutsu naturally, which makes them very coveted.
In Part I, Kakashi claimed that the Byakugan had better perceptive abilities than the Sharingan. While this is true to a certain extent, the Sharingan is superior to the Byakugan. The Byakugan is mainly used for taijutsu, while the Sharingan is used for ninjutsu, taijutsu, and genjutsu. Furthermore, it has the potential to evolve into the Mangekyou Sharingan.
2 God of Shinobi
The title of "God of Shinobi" is a significant one and the first person with that epithet was Hiruzen Sarutobi, who served as the Third Hokage of the Hidden Leaf. Hiruzen was a genius and throughout his life, he mastered all the known jutsu. Even in his old age, Hiruzen managed to hold off Orochimaru and the reanimated First and Second Hokage.
Again, this was changed by Kishimoto. The first person with this epithet was actually Hashirama Senju, the First Hokage. By adding this information, Kishimoto managed to make the rivalry between Hashirama and Madara even more interesting.
1 Uzumaki Clan
Naruto is the son of Minato Namikaze and Kushina Uzumaki, so normally he would have been named Naruto Namikaze. However, due to safety concerns, Naruto was given the Uzumaki surname. The decision helped to prevent any harm coming to Naruto, as many people would have tried to kidnap him.
At first, the Uzumaki clan did not seem special, but Kishimoto later added more information, which confirmed that the Uzumaki clan was once the most powerful clan in the world. This was used to make a connection between Naruto and Asura Otsutsuki.