One thing is DNS changes take time to happen, and it sounds like you haven't waited for these to propagate.
bad_httpd_conf means that traffic is arriving for siteX and the server is not configured to serve siteX. What's happaned is that the instruction has reached the server telling it not to serve the site anymore, but he DNS change to re-direct the traffic still hasn't yet propagated.
An easy way to understand this, think of the IP address as a phone number and DNS as the phone book. When you told the dreamhost panel you wanted to replace your site with your tumblr, that changed the sites phone number. The phone book tho, still has the old number in it. Propagation is like the phone company publishing and distributing a new phone book every 4 to 8 hours. While you're getting that bad_httpd_conf error, you're just waiting for the new phone book to be distributed.
the bad_httpd_conf error can exist for other problems, but this is mostly likely why you are seeing it. Remember anytime you make a change that effects DNS it will most likely take 4-12 hours for that change to be seen, and the change won't be effective for everyone at the same time, some could see old and some see new during this period (after all you can't deliver 7 billion phone books all at the same time).