I had a similar problem with information on a web site that I wanted to convert to an eBook for selling. To maintain the relevancy for the search engines, I scrambled most of the content by using BBedit to sort every line into alphabetical order. At the top of the page I put a note to explain that the content was now for sale in the form of an eBook.

You are right never to delete a page. I've spent years getting a good PR and I'm not going to throw away pages that have a top ranking!

Pages which are now just plain obsolete, I put Google "Adsense" code in, so they earn me money that way.