Never mind, I'm the one that's dense. It's been a long while since I've used those. Yes, the page that calls the include must have the unique extension - this is what tells the server that it need to look for and add any includes in the page.
So, you can use the JavaScript method, but I'd advise against it because it wouldn't work for people that have js disabled.
So, you can do a couple of things:
1. Have your host configure the server to parse htm/html files for includes. I doub't they will do this, but it doesn't hurt to ask.
2. rename all your pages with the shtml extension, then create new pages with the original names, but make them a single frame which loads the new shtml files. This will alllow you to keep your rankings - at least for now. Since the actual page that is loading is just a fram I don't know if the frame source will be indexed.
3. (My preference) rename all of the extensions to shtml. Then recreate the original pages but have them be a page with a meta refresh tag that will load the new shtml page (you should also include some text along the lines of "You will be redirected <u>here</u> after 5 seconds, if not click <u>here</u>"). You will also need to update the links within your code to point tot he shtml pages.
Now, all of your html pages will still be indexed and users will only get the redirect page once when they hit your site from a search engine. Over time the shtml pages will get indexed. You can also put a tag in the header (google for it) of the html redirect pages that will tell the bots not to index them anymore, but make sure they index the linked page! Again, google for more info on how to do this.
Eventually you can check you logs to see how many hits you are getting on you html redirect pages. Once the level is down to an acceptable level you can remove those pages if you wish.
Michael J