Interesting reading and certainly the story has a lot of potential. Certainly you need to flesh out some points but since this is more like a draft and I understand that.
One important thing that you should keep if the first person narrative. You have done well with the girl lossing the traits. Now, you should keep that first person narrative when you talk about the girl gaining them.
That way you can have two narrators and people can read from the perspective of both girls.
You are also doing well explaining the changes in the girl and of course, how those affect them. In the case of the first girl who she panics with her clothes doesn't fitting them and how lose or tight they are in some areas. The same you can do with the second girl explaining how good her clothes fit her now.
About the story, I can imagine that you started it in some point of the future since the firt girl mentions that this will be another night in which a part of her will be stolen and she also know who gets benefit from it. So that can be a nice hook to get people interested to see in which point of lost she is when you begin the story and what had happened so far.
About the transfer method, that is nice and it seems like a time period activation, so after sometime the transfer will take place without any girl influencing on it.
I wish you the best luck with your story my friend and if you need more feedback or comments once you work some more on it, I will be more than glad to help you out.
Ah! You can bet that I will be looking foward for more of your story.
Max