Forum Replacing old site with new CMS. Google redirect and/or archive suggestions
 

What the best thing to do here?

I have a website and phpbb forum that has been in the root directory http://mysite.com for 10 years. Its indexed deep and busy. I am going to replace it with the new vB4 CMS.

Should I simply take the old site and archive it to a new directory ( http://mysite.com/archive ) and replace it with the vB4 CMS? Having the new CMS in a sub directory isn't an option because of all the people that go directly to my root directory ( http://mydomain.com ).
Will google continue to reindex the new CMS and eventually I will be back on track or is there some magic I should be doing?

That being said... doing a redirect 301 concerns me because I would have duplicate content however, different urls if I keep both CMS indexed. I've read this can be a bad thing for google.

Any suggestions would be great.