How does Romeo change from the start of Act I to the end?
Since Romeo has only three days in which to effect any personality change, little happens within him. For the most part, he remains impetuous, emotionally involved with fate--
Some consequence, yet hanging in the stars,
Shall bitterly begin his fearful date...(1.4.13)
and later, yet feeling the control of fate in his life, he declares, "Then I defy you, stars!"(5.1.24)--and idealistically in love. Perhaps the only change in Romeo is the alteration of his attitude toward the Capulets: While he has hated them as a true Montague should since the families have long been involved in a vendetta, after having married Juliet, Romeo softened in his feelings toward the Capulets. His profession of love to Tybalt in Act III--
I do protest I never injured thee,
But love thee better than thou canst devise
Till thou shalt know the reason of my love.
And so, good Capulet--which name I tender
As dearly as mine own--be satisfied. (3.1.68-71)
--attests to this.
Still, there is some modification in Romeo's personality. Prior to his secret marriage to Juliet, Romeo was known as a rather reasonable young man. But, his reactionary slaying of Tybalt in Act III, along with his rash murdering of anyone who enters Juliet's tomb indicates that in his desperate love for Juliet, Romeo lost his reasonableness. Truly, in Romeo, "The violent delights have violent ends."