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Note: on one of the pages it is the #2 result for my main keyword (right behind the homepage).
For a site that I recently helped out with I merely put <meta name="robots" content="noindex,follow"> on every old page, and made all of the navigation on the old pages point directly to the new pages (so that first click anywhere on the old site took you onto the new site). It took Google nearly two months to drop the old pages, and list the new ones in their place. The old pages will stay up until all the incoming links have been amended to point to the new site.
Google cleanly delisted all of the pages of the old site on 2003-06-15, and added all of the new site into the index the very same day.
ErrorDocument 404 [YourHomeUrl.com...]
sends all page not found to my home page. Works fine for me!
::Google or other SEs would see the 404 status and use this as a flag to remove the page from their index.
I did that and it took about six months before Google deleted the dead links!
If you do just that, and nothing else, you will get a duplicate content penalty. Google will list only one copy of the page, and not the other. Thing is, you'll have no control over whether they list the old or the new page.
To ensure they drop the old pages, you must either flag them as robots: "noindex" or else remove the content and put a 301 redirect in.
If you can set up 301 permanent redirects for the old to the new, then that would be the preferred method. If not, then definitely get a correct 404 page in place so that the dup content issue is addressed.
Google is pretty good about merging duplicate content and dropping dead pages after an update or two. Between now and then, you may find yourself with some dual listings (indented) that show both the new and old pages until Google sorts things out.