I don't think they'll do that. She's not going to change into anyone else. I don't think she will even become a female "Thor" as such. She will be a hammer wielder who is worthy to wield the hammer, but will stay essentially Jane Foster. She might get some armour, but her name is not suddenly going to be "Thor". Thor is Thor's actual name, just like Steve Rogers was Steve Rogers's actual name. It's not like the original comics where we had Dr Donald Blake who was an ordinary human who changed into Thor with a physical transformation. Thor is Thor all the time. Even if he became unworthy to wield the hammer anymore, he's not going to stop looking like he is or being who he is. On a different note, I wonder if we'll get Natalie with bare arms like the comic version, or whether it will be covered up like movie Thor's armour.