Whaddya do?
If you are hosting on an Apache server, there is this cool configuration file named .htaccess which can give directives to the server for handling everything from hotlinking to error handling and more.
This can also be used to handle redirects. What can save your SEO is finding all your important existing pages, and, using .htaccess, redirect them with a 301 redirect (which means it's a permanent move) from the old file to the new file.
redirect 301 /old/old.asp http://www.you.com/new.aspx
The above will send the visitor seamlessly to the new file and tell the browser or crawler, that this is a permanent move.
On IIS servers there are most certainly ASAPI filters that can handle this same situation, though I can't point to any for sure. On other platforms you can just as well keep the existing files and make them only contain header redirects as well.
