Is it a bad thing if someone writes a story in the third point of view, then he changes the whole story into the first point of view where the protagonist himself tells the story?
I assume you're talking about the author, while revising their work, making that change.
And that's perfectly fine. It's a good idea, in fact, to try out different points of view. Try writing a section in first person just to see how it makes the story sound. Or read the whole thing to yourself as if it were first person. It might improve it or not. There's a lot of experimentation in writing.
Most first person stories, as well as many third person stories, stay in a single character's perspective. In some point of view strategies, however, you can shift perspective. For example: ... Changing perspective also demands the reader readjust after being so invested in the character whose perspective came before.