The first episode of 'One Piece' premiered on October 20, 1999. Today, 24 years have passed and the anime is still streaming. Throughout this time, the aspect ratio, resolution, animation quality and design style improved with notable changes to this day.
The first change in the animation of One Piece occurs from episode 207, where the aspect ratio changes from 4:3 (640x480) to 16:9 (1280x720), something very noticeable, going from a square to a rectangular size, encompassing an extensive panorama and improving its quality.

The resolution of the anime as of episode 207 changed from 360p to 720p, which is a better quality and can currently be found in 1080p. That is why on some websites you find One Piece episodes 1 to 206 in low quality, but on current streaming platforms, the anime is available in HD, FHD or UHD, because the episodes were remastered.
When does the animation style of One Piece change?
If we talk about the change in character design or animation style, this occurs from episode 517, which is where the Fishman Island Saga begins. After the TimeSkip that lasted 2 years, Luffy's crew reunites, with new appearances. due to time elapsed or their respective training.
Another change in the style of the characters is seen in the Wano Arc, from episode 890 onwards, the crew's clothing is traditional focused on Japanese culture, where samurai, ninjas, sumos, etc. existed. It also refers to Japanese food, music, dance, history, etc.