Whenever you make changes to a web site, one of the most important considerations should be how to use “redirects” to alert the search engine to your changes to avoid having a negative impact on your search rankings. Whether you’re moving pages around, switching CMS platforms, or just wanting to avoid duplicate content and PageRank dilution, you’ll want to employ redirects so as not to squander any link juice (PageRank) that your site has acquired. There are multiple ways of redirecting, and it’s important you get it right if you want the SEO benefit without risk of falling outside search engine guidelines (such as is the case with “conditional redirects”).

Read more here: Redirects: Good, Bad & Conditional