Selected answers from the Dumb SEO Questions Facebook & G+ community.
Perry Bernard: The www version is a different website in Google’s view. As with any subdomain. You should follow all the recommendations you might use to migrate to any new domain. If your site had a mix of use and already redirected one version to the other, then probably the impact won’t be too bad. However if Google never crawled any non-canonical versions before, the impact will be greatest.
ქუჯი თáƒáƒ“უáƒ: Perry Bernard thanks for your advice. There is no mix of use yet but we need the final domain to be with WWW. Google never crawled and I am interesting how much does it take approximately to restore the existing ranking positions.
Cristina Ingram: Perry Bernard agreed with Perry Bernard, 80% percent works, if you do 301 smoothly
Richard Hearne: It generally won`t hurt (unless you`re doing something entirely weird in your set-up), but I`d say it`s unlikely to make a lot of difference overall. Your rankings are very unlikely to change due to this one change.
However, it`s one of the "nuts-and-bolts" tactics that good sites will always observe. I`d always try to ensure resources are only available on a single URL as much as is possible, and this is one way to reduce the possibilities of multiple URLs existing for the same content.
ქუჯი თáƒáƒ“უáƒ: Richard Hearne thanks for your answer. However if I redirect to www its won`t be possible to have only available source, because people will mostly type without www. What other cons may www have?
Richard Hearne: None. The only con might occur if you have resources available on multiple URLs/Domains (www vs non-www). Always better to have a resource respond to a single URL only. Just set up your redirects now, and move on to something more beneficial for SEO.
Hi. I have a question - how much will redirecting from mydomain.com to www.mydomain.com affect SEO? 
