Page is a not externally linkable
SEOtop10 - 10:19 am on Nov 12, 2005 (gmt 0)
For new sites, consider using extensionless URI as W3 recommends.
Whether you change to .php or .asp or any other extension/technology, when you have .htm or .html listed in the search engine, just continue using the same extensions and change the engine processing this extension to the appropriate one.