Changing my Website Theme & Design

I am going to change my theme & design. I will first do it on a different server - than I will replace the the old design with the new one.

I have blog posts on the old design - How I can merge it with the new design?
Also, Will it affect the websites rankings?

I've changed themes on my site before without it impacting rankings. What you will notice is that Google and the rest index various theme files, so when you change to the new site/theme/design, you will have an uptick in 404 errors for a while. More so if you also change up content strategies or silo architecture.

Sadly, there is no easy way to remove the removed pages from the indexes anymore. Deleting the sitemap and uploading a new one in GWT or Bing's equivalent is a nice start, but it's not the magic pill to make the 404'd pages go away.

Yes, it can cause changes in rankings, be careful.

Are you using wordpress for your blog?