How much should an article be altered to avoid Duplicate Content

I have 20 articles for my Green Tea website. Each one deals with a different aspect of green tea.
I would like to promote each article on 5 different Article Directories. This would give me roughly 100 backlinks from article directories.

Rewriting my articles is hard work.

How much do I have to alter my articles to avoid Google's Duplicate Content? (Whether or not it is proved to exist - I do not care about that debate, I'd rather be safe than sorry).

Is changing 1-2 words every sentence adequate? Or even 1-2 words every few sentences? Or must I do a major overhaul -- alter entire paragraph structures?

Question two: What if I only copy/paste an article on 2 different directories. That way there are only 2 duplicate articles out there, and I alter the articles every 2 postings? That way there is only ever 2 versions of the same thing. Would search engines still pick this up as spam?

 

make changes in paragraphs and sentences. Also make new paragraphs. After all check with copyscape.com. Thus you can go away from duplicate content

 

Best way is to check it with copyscape.com and edit it till it passes over there.

 

make a summary of it.. and don't get the idea of just one article..you can still search for other related topics..