If you are changing the URL, you can copy all the page information that you want from one to the other, but to update crawlers, bookmarks and the like, you have to indicate a proper redirection.
A 301 redirection is not necessarily bad, is not just that some of the "juice" from the original is going to pass to the new one, but also the preferred way to indicate what you mention. At least as far as we can tell from your description.
Remember that ranking is achieved by many elements, not just one and although changing the URL means that you have to get the ranking for that new page, if it's related to the old one as a substitute, it's the best way to start the process.
Just be sure that you do all the other steps correctly, create a good page, use all the information and metainformation required, don't stuff keywords, have appropriate links, etc.
In the new page, you may have a little but descriptive text telling the users the reason for the change, at least for some time, that way your users will update any manual bookmark and will feel more confident with your site. Of course the redirection should be on for as long as needed, a month, a year, ...
Finally, you can talk with the client and explain the situation clearly so he can decide what to do with all the information. Don't present any scenario as worst, just present the facts.
