The sort of people are Portia, Antonio, Bassanio, Shylock, Jessica at the beginning of the play? What caused the changes?

Of the five characters you mentioned, I would say all of them but Portia are dynamic.  It would be difficult to prove that Portia undergoes any significant changes.  At the beginning of Merchant, Portia is independent, wealthy, and clever.  She remains the same at the end of the play.  The other four characters change in the following ways:

-Antonio begins the play as a melancholy, wealthy merchant.  He is arrogant, insulting, but blindly loyal to his friend (Bassanio). Shakespeare never explains Antonio's depression from the beginning of the play, but it seems that Antonio ends the play still in a somber mood. After his near-death experience, thought, I believe that Antonio has been humbled and would be more careful about whom he chooses to do business.  The extent of Antonio's change depends upon the play director's interpretation of Antonio's courtroom scene.  By reading the play, I think that Shakespeared intended for Antonio to come across as a clever businessman who "resentences" Shylock not to show mercy or sympathy, but rather because his sentence benefits Antonio.  In that sense, Antonio is not a very different character at the end of the play.  But, if one sees Antonio's new sentence for Shylock as being merciful and almost an apology on Antonio's part, then he/she could argue that Antonio has become less prejudice because he realizes what his insults toward Shylock have done.

-Bassanio matures quite a bit throughout the play.  He begins as a fun-loving, likeable, free-spending young man.  But, because of what he puts his close friend Antonio through, he develops into a more responsible and thoughtful husband and friend.

-Jessica begins the play as a rather rebellious, wild daughter. As the reality of what she has done to her father and in turn what he has done to others, she is not quite so resentful toward him at the play's end. While she still loves Lorenzo, her speeches, especially in Act 5, demonstrate that she thinks somewhat like her father and might feel some regret for leaving him in the manner that she did.

-Shylock undergoes the most change.  At the beginning of the play, he is a cutthroat businessman. After his humiliating defeat during the trial, he leaves the Duke's Palace, a humbled, sick old man.  His change is the result of his insistence on exacting the letter of the law and then having the letter of the law thrown at him.