Problem
I used to have a web server running IIS and BlogEngine. When people come to my new site looking for an article they will get a nasty error. For one the site is now PHP. I wanted to remap all the files from the old URL’s to the new URL’s.
Old URL
http://www.devclarity.com/blog/post/topic.aspx
New URL
http://www.devclarity.com/index.php?s=topic
I figured I would just put a redirect in .htaccess, but little did I know that the web server sees the .aspx extension of the old URL and processes it on the IIS server. Thanks a lot Rackspace Cloud 
Solution
I created a Web.Config file to send all errors to a page called Redirect.aspx.<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<system.web>
<customErrors defaultRedirect="redirect.aspx" mode="On">
<error statusCode="500" redirect="redirect.aspx" />
<error statusCode="404" redirect="redirect.aspx" />
<error statusCode="403" redirect="redirect.aspx" />
</customErrors>
</system.web>
</configuration>
Then, redirect.aspx takes the URL and modifies the aspx extension to a html one and redirects the new link back to the page.
<%
Dim ASPXPath as String
ASPXPath = Request.QueryString("aspxerrorpath")
ASPXPath = ASPXPath.Replace("aspx", "html").replace("-", "+")
%>
<html>
<body>
<h3>Please wait, your content has moved</h3>
<script>
window.location='<%=ASPXPath%>';
</script>
</body>
</html>
Old URL:
http://www.devclarity.com/blog/post/topic.aspx
New URL:
http://www.devclarity.com/blog/post/topic.html
RewriteRule ^blog/post/([\w-\+]+)\.html$ /index.php?s=$1 [R=permanent,L]
Result
http://www.devclarity.com/blog/post/Backup-Mover.aspx now takes you to the remapped Backup Mover page on the Wordpress site.
(Of course, this post is new so if you use the old backup-mover link you will see this page first with the backup mover article under it
)