When redeveloping your existing website, consideration must be given to maintaining the SEO reputation of your existing website. Your domain name is a valuable asset in your ranking for search engines.
Over time your domain gathers reputation, which improves your ranking potential in search engines like Google. This ranking applies to all of the pages on your website. When you redevelop a web site it is not uncommon to change the names of individual pages. If you do change your web page titles, then you should create 301 redirects for the pages that have been renamed to redirect either to the new equivalent page or an appropriate replacement page.
301 redirects can be supplied to search engines in your web sites .htaccess file.
Redirect 301 /index.html http://newsite.co.nz/index.php