This is a concept that I have thought about a lot, but I think it'd be extraordinarily hard to write well for it, and there are precious few stories about controlling more than one body.
I really like your ideas about personality and how a story like this could show how personality works. It really does make sense to me.
I'm going to brainstorm a bit myself here about similar ideas as well. I hope I don't come across as criticizing negatively here. I'm just posting my thoughts and trying to help make a great story.
A similar idea I have had for starting a story may be having one "Jon" being the original and one in an additional body, treated by the world as twin brothers. Alternatively, he's dealing with being the opposite gender as well, I think it may be more logical and easier to just have his current body and his "twin sister's" body.
Having only two bodies would allow easier distinction between multiple personalities to the reader than having four bodies, which is even harder for a reader to wrap their head around. It's hard enough for most stories to have more than one or two main characters, let alone having one of the main characters have multiple personalities, so I think that it may be best to stick with fewer.
Your idea about the theme of change sounds awesome. Right now, this branch feels like it is very much so based on change. I would love for it to continue based on this theme, and I think that it currently is at a strong point for continuation. Doing this from primarily Karyn's point of view makes it very interesting to me.
The final few paragraphs of the most recent chapter are really interesting and contribute very well to the theme of change that you stated in your post. http://fb.countd.eu/cgi-bin/fbstorypage.pl?page=161010004732no1&path;=nospamstory
