I have never been able to get a North Star cherry tree to overwinter where I live. Rather than replace it with a Dwarf Juliet, I very strongly recommend that you get an Evans Bali for a replacement. They are also a natural dwarf tree and where I am they have survived winters that have killed elms, other cherry trees and apple trees.
I currently have 3 and they are all loaded with cherries.
That stupid northstar was 14 years old and only succumbed to a disease the trees by it are apricots and they are going strong. I'm in wabasha county. And is that Evan's Bali a sweet cherry? Just would like a change.