I have some pages on my site that are plain HTML pages, but I want to add some ASP .NET type functionality to these pages. My concern is that if I simple rename the .html page to .aspx that I will break links, and lose SEO, and so on.
I would think there is a "best practice" for how to handle this situation.
You could use ISAPI rewrite to redirect the .html urls to the .aspx urls.
The idea here is that you rename all the existing pages to .aspx, but have all incoming requests handled as a permanent redirect (301) to the new .aspx pages. This means all incoming links to *.html will find the correct *.aspx page.
If the new ASP.NET functionalities is minor, I would recommend to include IFrames inside your HTML which links to the new created ASP.NET pages containing your minor dynamic changes.
If you control IIS - you could just map .HTML to the ASP.NET handler and run them as is. Or, map them to a custom HttpHandler and send a 301 code with the updated location.
Create your new pages on aspx, and just serve 301 permanent redirects from the HTML pages.
Search spiders are smart enough to realize the content has moved and will not penalize you.
Both Google and Yahoo also say that they parse a meta-refresh with no delay as a 301 redirect, so just do something like this:
<html>
<head>
<title>Moved to new URL: http://example.com/newurl</title>
<meta http-equiv="refresh" content="0; url=http://example.com/newurl" />
<meta name="robots" content="noindex,follow" />
</head>
<body>
<h1>This page has been moved to http://example.com/newurl</h1>
<p>If your browser doesn't redirect you to the new location please <a href="http://example.com/newurl"><b>click here</b></a>, sorry for the hassles!</p>
</body>
</html>
All this would work, but I think a lot depends on the content of the pages, the amount of programming you need to add to it, and the plans going forward. If they are valued by search engines,
In general, however, I think your best bet would be to create new pages and 301 redirect the html pages to the .aspx pages. Alternatively, you could do a URL rewrite to display the .aspx page while leaving the URL the same, but that is not a very scalable solution.