As far as I know it won't. Improving the body content for "better, richer and longer" might be even the solution to push more the page on Google SERPs.

Also, creating a new page for that page would also be better. Let that ranking page stay there and then just build contents and new links for your new page - put the same keyword you use on that ranking page to your new page and link it to. Build quality links for the new page and you will definitely see the results on both page pushing each other way up the SERPs.

It could affect your rankings negatively if you drastically changed the keyword density or made the post not as related for the keyword in question.

In your case, I would leave the current content, but build on it and add to the page without taking anything out. I have had articles that were short but on page 2, went in and edited the page added a few hundred words of related content and it pushed me on to the first page. So you should be good if you build out the page further.

In my experience, Google likes to see these pages edited and built on, because it increases the relevance and "freshness" of the page.

As long as you don't do anything stupid (I'll let you dictate what is or isn't stupid) then you will be fine. Just ensure that the URL doesn't change and your fine - personally I don't think keyword density has much effect on your rankings, so try not to worry about it too much.

It will almost certainly affect your page's ranking. In what direction, it's hard to say. If anything, I'd just add some more content below it as opposed to changing up stuff that's already there.