Keeping link juice from existing clients site

Hi, I have a general question about sep and link juice. I am not an expert by far with seo so hope i can keep this simple.

I am about to migrate my new clients site to their existing server under exactly the same url which means the existing site is going to be replaced.

The problem i have is the existing site currently sits on page 1 for their keyword which is ‘door sets’. the new site has a completely new page structure and new content so i am in the position where i need to keep as much of the link juice as possible considering the new site is completely new.

The question is… what is my best option to retain as much page rank as possible? Even if i only worry about the homepage for now?

I have heard to 301 redirect is the best option but how do i do that when the new site is replacing the last on the same server?

Thanks for any help in advance.