Replacing a #1 ranking page with another page

Thanks for adding me to the group Ammon. I have a question about replacing a #1 ranking page with another page - can it be done with confidence? My client`s most popular landing page is an article about a product they supply. Its ranked well for a long time (18 months) and consistently drives good traffic. They have written a `better` page. It has more info including diagrams and is their preferred page to rank in place of the existing page. I don`t think that linking from the old page to the new page is a good idea as the user experience will be diminished (its similar but unique content). The new page doesn`t rank for these target phrases, its not been optimised for the same phrases (as yet). I`m concerned about using rel=canonical from a ranking page to a non-ranking page as i feel that is bound to lose the ranking. My thoughts are that I should leave the old page and optimise the new page for the same phrases then wait to see if the new page can rank for that phrase. If it gets on page 1 then use canonical but then it might still lose the #1 spot? I`d suggest editing the page to make improvements to the copy but again could this risk the #1 spot? Any thoughts would be much appreciated.