How much has changed since the start of One Piece?
The anime moved to Bandai Entertainment since Episode 205, and then it started to run in widescreen and not 4:3.
Quality changes are just because this anime exists for about 15 years, so it's only natural for the video quality itself to improve over time.
Edit: In the anime, the change between old One Piece and new One Piece is alot more clear than in the manga. The quality of the animation started being alot better after One Piece moved to Bandai Entertainment.

Yes there are big changes.
Episodes 517+ happen in a time called "post-timeskip" and in "the new world".
Episodes 1-516 are referred as "pre-timeskip", because after episode 400 alot of big things happen, and they change the whole world of One Piece, but I'm not going to spoil your experience, I can just tell you this: enjoy the East Blue saga, expect big things and surprises, and have fun.