There are two options that I can think of off the top of my head. The first is to create another blog, just add "testblog" or something of the sort to the name of your real blog, or however you want to title it. Keep your test blog private, import the posts & comments from your real blog, and then play around to your heart's content.
The other option would be to temporarily make your blog private. The problem with that, of course, is that any visitors to the site who encounter the announcement that your blog is private will likely not return. It might also confuse the search engines if it was marked "private for very long, though I don't know.