Changing Domain Only – Same Hosting/Database

I have two sites that I started as subdomains. I now own the domain name and set up a temp redirect to the old site. Now is the time to move to the domain.

A few questions: I don’t want to lose all the links I have from forums, articles, facebook, twitter, etc. so couldn’t I leave the old site intact? I would add a post saying “For new content go to …” with the new site domain.

I would think that google might look at the posts on the new domain and see those as duplicate content, but the new content would be okay.

I’d like to make this move as simple as possible.