Hi,
If you changed your filenames then much of your hard work optimising the site when it was static will be lost.
For example if you have a page called cheap-web-design.html that has a page rank of 2 and 30 inbound links if your new page is called cheap-web-design.aspx then you'd have to start again creating the inbound links and telling google about the page.
You need to create a 301 redirect in an .htaccess from every old page to the new page.
If the filename is exactly the same except for the change in extension from .html to .aspx then you can probably do it with one rewrite rule.
Ideally you could tell your web server to handle .html or .htm as if they are .net files and change all your files back to the original file names but that would be a lot of work!
Chris.