Google has specific advice for anybody moving a website:
- Think about making CMS/Layout changes and URL changes in stages
- Use 301 redirects
- Change all the links (both internal and external) that you can
- Retain control of your old site for at least 180 days (but I would recommend indefinitely)
- Use the Change of Address tool (However it doesn't work for subdomain moves, if you try it states "Restricted to root level domains only")
- Add the new site to webmaster tools (WMT)
- Ask googlebot to crawl the new site using fetch as Googlebot in WMT
- Submit a sitemap
- Check for crawl errors
- Keep an eye on the number of pages that WMT reports as indexed
A few years ago I moved a subdomain to its own website, just as you are trying to do. I used 301 redirects. As soon as Googlebot found that the site was moved (starting in a couple days), I lost 90% of referral traffic. Google didn't send significant amounts of traffic to the site for eight months. Suddenly, after 8 months, traffic came back at the same old levels and continued to grow from that point on.
I understand that the situation is much better these days than when I did it. I've seen webmasters reporting two to four weeks of traffic loss rather than many months. Still, expect to lose some traffic for some amount of time.
It would be nice if Google were able to support moves like this without any penalty. You can move from one domain to another with no penalty if you use 301 redirects and the change of address tool. I wish that they would support sub-domains in the change of address tool.