don't do it ever !
A general rule is to never ever expire URLs. At the minimum redirect all pages to the new location. It is not so much the search engines, that hurt, but all the other sites linking to you and all the bookmarks that people have (hopefully) stored about your site.
A custom 404 pages is not a solution! First it does lead you to one landing place and second it does cloud your ability to find real broken links in your site. Because you have all these broken external links.
If you are good, you filter all pages for redirected from your logs and send an e-mail to the web master of the link source asking for changing to the new page.
Good luck
K<o>
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