I have two Drupal sites with different content-- let's call them Apple and Orange. I recently developed a much more sophisticated third Drupal site-- let's call it Tree. For a large number of reasons, the content on Apple and Orange is useful for the users of Tree, so I want to move the content to Tree.
However, much of the content is out of date. (This whole process took about five years.) To update the content, I will rewrite it one article at a time myself. Now here's my question: if I move the articles one by one (as I rewrite them) and then redirect the old articles (using a 301 redirect) on Apple/Orange to the new site on Tree, will this have a huge negative effect on my search engine rankings? Is there a good way to redirect among sites when they merge like this, or would I be better off keeping the old articles on Apple/Orange and simply linking them to the new, rewritten articles on Tree?