You mean your site was already hosted on an account at Dreamhost? If so, you'll need support to do the switch for you.
It shouldn't be a problem unless the accounts can be linked somehow & both used promo codes.
If you mean it's hosted somewhere else... then what's the error you're getting?
Save up to $96 at Dreamhost with ALMOST97 promo code (I get $1).
Or save $97 with THEFULL97.