I changed my file extensions, but google hasn't changed
Have you redirected from old .html filenames to new .shtml filenames? You have to do this - Google will pick up your changes faster (because it doesn't have to reopen all your pages), and it will carry PageRank from old URLs to new ones. There are a million descriptions on the net on how to do this.
But with that said, can't you just store the .html filenames and do some trickery with .htaccess (or the equivalent, you don't say which webserver you're using) to make the .html files comply with SSI directives? It:
AddType text/html .html
AddHandler server-parsed .html
should do it for Apache.
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If your site is getting some traffic, then there is a good chance that Googlebot has already been visited. My site is small and I have to go around daily. You can be sure that Google already knows your new URLs.
The point is that Google only updates its indices every 3-4 weeks or so. There are a few exceptions to this, such as regularly updated blogs and forums, where new topics and posts are received very quickly. But it can usually take several weeks for your new URLs to hit the main search index.
Oops and tip: if you are rewriting the rules to redirect old .html addresses to your new .shtml addresses, make sure you are using 301 redirects and not 302 redirects. With 301 redirects, Google will assign the "link juice" that the old url has to the new url. With 302 redirects, it doesn't, and you could lose your pager.
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