What percentage of an old post can I change without lose rank?

[DSG]

A few months ago I updated some old posts of my site (Wordpress), to improve old content, with new pictures (better quality), better content, new info about product, and links to our review of this product (newer posts).

Most part of the content is this old posts were new, so I "deleted" the old text but replaced the new one with another of better quality.

I suppose it will help new post to improve rank, but I los thousands of daily visits, because old posts ranks worse since the change.

What do you think? What happened here? Maybe I shouldn't update the whole text, only add a link to new content?

Thank you!

[IanTheScot]

If your posts are getting good traffic and rank well already, I recommend writing a new post and putting a link at the top of the old post saying that you have created an updated article.

You can never be sure what the "magic sauce" was that made Google like your original article.