Reyzer, there really is no risk when you move from http://domain.com to http://www.domain.com on the same domain. If you were to move from one domain name to another, that would be different--and you'd want to use Google's Change of Address tool to tell them that you're moving from one domain name to another.
But, when you move from one subdomain to another subdomain (which you are doing in your case), there is NO loss as long as you properly implement 301 Permanent Redirects on the pages. Google has been very public about this, and as long as you move using a 301 redirect and it's on the same domain name, there is NO loss whatsoever.
The same goes for moving from http:// to https:// as well, as it's the same domain name.