Both are possible. Because it evidently happened within the last month, I would probably use "you've changed".
I haven't seen my wife's friend Annie in a month or so, and she dyed her hair platinum blond. I say to her:
"Hey, love your new hair color, you've changed it to such a bright shade!"
"Hey, love your new hair color, you changed it to such a bright shade!"
Are both useable here? What would a native speaker say?
If the comment is being made on seeing it for the first, then it's the present perfect for me too.