The question presupposes that the main characters have changed, but I would question how much, with the exception of Shylock, they actually change within the play, and how much the play ends with the beginnings and promise of their transformation.
Certainly, Shakespeare leaves us with strong suggestions of change. The play begins in mercantile Venice, with Antonio's first line being about sadness:
In sooth, I know not why I am so sad.
It wearies me; you say it wearies you.
It ends with most of the main characters in Belmont, place of love. They are all happy, even Antonio, who there learns the news that all three of his ships are safe after all. He says to Portia, after hearing the news, that she has given him both "life and living" (or income). After what he has been through, he is no longer talking about sadness, perhaps having a new appreciation of the fragility of both his mortal body and his wealth.
Shakespeare also intimates that Portia, Nerissa, and Jessica have gained new confidence from...
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