Many sites will undergo major changes periodically, but in most cases the same content will still be available from each iteration. Search engines typically rank pages on a per-page basis,where the URL is the unique identifier, so changing URL means losing any rankings that page may have previously had.

Sites should not move to new domain names often, but if they do, a 301 redirect is the best approach. Note that it may take up a year for a relocated site to rank well again in search results.Disruption to search engines from site changes can be greatly minimised by following the advice given earlier on URL schemes and correct use of redirection codes.

Another issue is that of sites that are available on multiple domain names. Best practice is to redirect all alternate names to a common domain using 301 redirection. Failing to do this can cause each of the domains to be indexed separately and to compete against each other for search engine rankings. In a worst-case scenario, the pages of the site may be placed into “supplemental indexes”, an area where search engines such as Google place pages that appear to contain no original or useful content.

This includes normalising whether your site’s URL has a www prefix or not. Search engines may see http://www.example.govt.nz and http://example.govt.nz as different sites, so it is best to choose which is the preferred form and issue a redirect to visitors using the other.

For more information : e.govt.nz/resources/research/SEO.pdf