I would argue that her attitude toward Juliet doesn't change throughout the play. I think that the nurse always wants what is best for Juliet. She wants Juliet to be happy, and she wants her to be well taken care of. What does change about the nurse is how she interacts with Juliet. At the beginning of the play, the nurse is quite comical and very friendly with Juliet. The two verbally spar with each other, and the nurse speaks some fairly raunchy jokes. The nurse encourages Juliet to take the risk with Romeo, and she even plays a major part in organizing the wedding. The nurse is clearly taking active steps to make sure that Juliet gets what she desires, and she does this knowing that it would make the Capulet family furious. Even in act 3, scene 5, the nurse defends Juliet's wishes to not marry Paris, and she nurse intervenes on Juliet's behalf when Lord Capulet is berating his daughter. Capulet turns his rage on her, and he verbally attacks the nurse. It's also possible that he physically attacks her, too. It's at this point that the nurse tells Juliet to forget Romeo and marry Paris:
Faith, here it is.
Romeo is banishèd, and all the world to nothing
That he dares ne'er come back to challenge you.
Or, if he do, it needs must be by stealth.
Then, since the case so stands as now it doth,
I think it best you married with the county.
Oh, he’s a lovely gentleman.
I don't think that the nurse has changed her opinion of Juliet. I think her opinion of Juliet's situation has changed. The nurse can no longer treat Juliet like a little girl. She's a girl who is married and is going to be married again, by her father's order. The nurse is advising Juliet to take the safest option and protect herself. Paris is financially stable, hasn't been banished, and is accepted by the Capulet family. He's good-looking. too. The nurse believes the time for taking risks is over—they tried, they failed, and now it's time for Juliet to grow up and make the smart decision.