How does Shylock's character transform from the start to the end of the play?

The answer to this question, as with anything regarding drama, depends somewhat on the actor playing the role. On film we have seen, from the late 1970s forward, radically different portrayals of Shylock by Laurence Olivier, Warren Mitchell, and Al Pacino. Modern productions have correctly, in my view, emphasized Shylock's unfortunate position in being victimized by the bigotry aimed at him. Initially, he is embittered but still shows a degree of self-confidence that enables him to "bear with a patient shrug" the insults and outrages leveled at him. He knows that Antonio and the others are hypocrites and that their discrimination against him is unjustified. Within the context of the Renaissance world (and unfortunately more recent times as well), Shylock has no recourse but to seek retribution:

And if you wrong us, shall we not revenge? If we are like you in the rest, we will resemble you in that.

But this ends up Shylock's undoing. In emulating the Christians who, in their own hypocritical way, endorse vengeance as a legitimate recourse, he dooms himself.
When his daughter Jessica elopes, Shylock shows an emotional loss of control, understandably, which contrasts with his earlier self-assurance. In the courtroom scene, when he has been defeated, he is reduced by his adversaries into saying, resignedly, "I am content." Again, much of the finality of this statement, and how it represents the changes Shylock has undergone, depends on the actor playing him and the way he speaks that final line.