In the manga, after Naruto left with Jiriya, they went to the Kakashi Gaiden, and after that, it had Naruto's return in PART 2, which is 2 & a half years later.


About the changing animation. Naruto produces 1 episode a week. The main artists are easily overworked at that rate, and so they substitutes. They work on one or two episodes, and then it's back to the main.