Naruto's First Major Retcon Was Its Best

screenrant.com:

With 720 episodes spread across 16 years, the story of Naruto has met dozens of retcons along the way – but the first significant one was its best. What was initially the journey of a lone ninja struggling to be recognized and create bonds, Naruto has since evolved into a lore-focused epic in which every arc has stakes higher than the last. For that reason, much of what had been established in the early Naruto canon had to change, leading to many significant retcons over the years.

Many of these retcons started in the transition from Naruto to Naruto Shippuden. The three-year time jump not only saw Naruto and his comrades get older but also expanded the scale of the story and brought in a significant amount of fresh lore. One such addition was the concept of the Tailed-Beasts or Bijuu, the nine creatures made of pure chakra that the Akatsuki was after. For most of Naruto Shippuden, the Akatsuki's quest for the tailed beasts was the show's main storyline, and it was thanks to this new concept thatNaruto made its first significant retcon.

Related: Why The Third Hokage Didn't Raise Naruto (Despite Kushina’s Wish)

In Naruto Part 1, Gaara's powers are said to come from the demon Shukaku, named after the Japanese tale Bunbuku Chagama about a tanuki that turned into a teapot. It was also established that Gaara's Sand Spirit was a corrupted sand priest, which relates to the Bunbuku Chagama tale. However, in Naruto Shippuden, Gaara's sand demon Shukaku was retconned to be the One-Tail bijuu and retroactively became "the First Tailed-Beast" aside from Naruto's Nine-Tails to appear on the show. Yet this retcon was actually one of Shippuden's best decisions as it made sure Gaara could be integrated into the overall Shippuden