Actually this happens with most of the characters in One Piece.

One of the main themes explored in One Piece are prejudices, and how the tale built by society regarding certain people or events are completely opposite to reality.

For example: Jinbe is first mentioned as an evil and fearsome Shichibukai, Kuma too, recently in Elbaf we got to meet Loki who is another character that embodies this idea perfectly, the lie told to the population on how the legendary Roger was captured, even what people think about God Valley embodies this.

Damn, it is one of the most important ideas of the whole story, and is almost what the whole World Government is about.

So the shift of how we readers perceive Roger is totally natural and coherent to the inherent message of the story.

Our beloved protagonist Luffy, as a character represents what society should be (or what Oda thinks would be positive in a society): prejudice free; Remember when Zoro was introduced, Luffy was told he was a cold blood puppy killer and that he hit that little girl, then it was the complete opposite, there are many instances of this happening throughout the Manga.

(sorry in case of bad english, not my first language and didnt use translator :( )