Congratulations, but need more info to get a sense of why you think this will freak her out. Will it affect her living situation? What kind of issues have there been? I know you might not want to "share," but it's hard to speculate.
She will probably be upset, no matter what. Kids have a certain schema in their minds of what a parent is, and when reality changes that, it's hard for them to assimilate the changes.