lets assume, I have a article that is ranked well (but not top) in search engine. Now, If I want to modify it a little more optimized, will the modification hurt on existing ranking in any ways?
It seems you are asking about putting more keywords in the article. We know from the latest Google Panda and Penguin updates, keywords in an article is least important.
If due to certain modifications in the article a user spends more time with the article or even if due to some of the links (internal/external) you have put in the article which is relevant to your article and as a result users spend more time than usual, is a plus for you
If you have modified the site in such a way that user tends to perform Call To Action on the site, then the Google sees and considers the article helpful to users and so Google will increase the ranking as well as the position of the article.
I will again say Keywords in the site is of least importance, now-a-days SEO is all about user experience with the website.
I believe that when the search engine next 're-crawls' the page it will 're-rank' it based on the new content. I don't believe it would damage the ranking as a result of making a change per-se, but if the content/changes aren't liked by the search engine (for any number of reasons) then that could adversely affect your ranking. It really depends exactly what changes you are making to your site. If its minor content changes then i wouldn't worry, if its a complete re-vamp of the site then it could work for or against you.
Hope that helps.
Google's Panda and their other patents are your worse enemy when changing ranking page content, I work on hundreds of sites and the best thing to do is improve a page by adding pictures, and text content but you should never TOUCH the key elements that Google and others is ranking you for.
Most SEO methods is unproven since only Google really knows how their engine works and its a very well kept secret on a lot of their factors specifically when it comes to changing page content. What you got to be aware of is changing the key elements as mentioned because Google can and most likely will think that those changes are due to the fact you want to RANK better and not necessary improve the page for your audience.
The Key Elements to avoid changing are:
1) Title Tag 2) Header Tags 3) Meta Description
As long as you add additional headers, pictures and text without changing the key elements your page should only improve or remain the same. Changing the key elements can resort in a negative effect and simply not worth changing if you are happy with the page rankings as it is.
It's worth mentioning I base this on the fact that Google have many patents about page changes that detect certain elements such as Header Tags, Title tags in such a way to to manipulate results or just to see what ranks better.
Google doesn't want Webmasters knowing exactly what the best tags to use and so forth, reason they don't want webmasters changing them often because well you get the point :P
In case you are working with your article in search engine, it is advisable not to change anything on purely experiment basis. Just focus on providing unique content with the useful information and try to optimize for long tail keywords and keyword phrases. Try to get more traffic through various ways.
If an article ranking at top position then it's a good for you. Here, I would like to suggest that modify your articles if there is a necessity to update important data or any modifications which occur is in technology or any other modifications which are helpful to readers then modify your article.
Finally don't modify it just for putting some targeted keywords which may back in result hurt your search engine ranking top positions for a specified article.