tbh if she's your stepmom, you'd still call her by her name too. You could call her both by her name and by "mom". Both would be okay. It'd be more of a building your relationship thing before you called them "mom" I feel like. That's how it seems to be in all the movies and shows and books I've watched and read where someone's parent remarries. I don't think a whole patch needs to be made to change it, but it's cool for the people who couldn't stand it the way it was I guess.
I guess one makes it more clear that she'd be your stepmom, while not definite? and one makes it vague between her being blood-related or at least raising you since you were much younger/adopted mother, or your stepmom that you never call by name.