url rewrite??

[steven s]

Don't quite know which section to ask. . .

I want to move a site from www.mydomain.com to old.mydomain.com.

Is there a rewrite so when people enter www.mydomain.com they are actually reading from old.mydomain.com but they see www.mydomain.com.

I don't want to redirect them, I don't want them to know that files are being pulled from a subdomain.
I want to appear that nothing has changed.