How to redesign wordpress.com site without publishing and lossing old one?

I have a wordpress.com site right now but I want to gradually work on redesigning it completely. How can I install a new theme and recreate the site without pulling the old one down til the new one is 100% ready? I hope that makes sense. You know how regular websites can have a website up and while they work on a whole new design. Then one day they just publish the new one. How can I do that?