search engines and 404

I had a web site which was well indexed with search engines.

However, I have recently taken the site offline, in favour of a new site
which has a different URL, structure and content.

In order to redirect people to the new URL, I deleted the old site from the
server and changed the htaccess file to set the new URL as the 404 error
page.

The question is... What effect will this have on the search engines when
they come to re-index the old site?  Any general wisdom about this sort of
thing?

Tim

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