copying pages

I’m creating a website for someone with pages (not blog pages). The owner wants to be able to replace some of the pages on a monthly basis. They want to prepare the new version in advance, but not make it visible to anyone, then to publish it when they are ready.

So, in brief, they want to make a copy of a page, edit it, then use it to replace the original.

What’s the best way to do this? Any ideas?