Configuring IIS to parse HTML like ASP pages is the easiest way to do this, if your hosting company will cooperate. Some may, but others will not reconfigure a server for a unique situation.
We recently did a complete rework of a site where directories and page names were all changed. It was basically a complete restructuring. The site was on a Linux server, so we set up the 301 redirects in the .htaccess file. It took Google about one week to change all of the old links in their data to the new versions. Other SEs took two to four months. From my experience, Google is the most responsive to 301 redirects.
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