Yolle on "New design, problem with mysql database"

I don't even know how to ask the question...
So - I am making a new design for my webpage, that has hundreds of posts and thousands of comments, photos.... I am making the new page on a new subdomain. I copied the wordpress files to the new subdomain and also the mysql database. In the time that I will be making the new design there will be a lot of changes on my current page, so I don't really know how I will make the transition back.
I will also be making a lot of new pages, so there won't only be new look but also new content on the new page... Is there a way to merge the two databases? Or?