I have an old site whose pages have .htm extensions. Does it matter in terms of seo if the same pages are uploaded with .html extensions? will the .html pages be considered new urls or different from the old .htm pages and lose the ranking they have?
Out of curiosity, why change to HTML? In my experience most site upgrades result in the loss of the file extension altogether.
Given a choice, I would try to preserve the .htm on any legacy pages to avoid creating any issues. If for some reason you have to change, then make sure you put up your redirects, and consider leaving your navigation pointing to the old .htm pages for a week or so to ensure Google finds the redirects... otherwise you can give yourself a temporary duplicate content problem.
The title of the thread implied an entirely different question with an entirely different answer.
index.htm or index.html
There the answer becomes: it doesn't matter in any way at all whatsoever because the visitor-- whether human or robot-- will never see the extension or even the word "index". If you have both htm and html, set a DirectoryIndex line to include both options, and make sure your index redirect includes an optional l.
The MultiViews aspect of mod_negotiation (assuming Apache) can serve up an available .htm file if the requested .html doesn't exist. But it's not something a site should rely on. Especially since I can't figure out whether the response is a 301 or direct serving of the variant file.
[mods note]i changed the "or" to "and" in the title[/mods note]
if you are asking about .htm vs .html extensions in general, you should pick one and 301 redirect requests for the other to the one you picked. if you are asking about /index.htm vs /index.html urls specifically, they should be 301 redirected to the directory root ("trailing slash url") and a default directory index document should be specified: