Changing the page extension could affect your rankings, but there are ways around it. For example, with my current project I'm converting an ASP site into PHP. I will have to leave a 301 redirect in the ASP pages for the spiders to get the new address. The only catch in my situation is the pages will have duplicate content (for a short while)if I do it that way. However the new design will greatly improve the site so I'm willing to take a hit on the ranks. I can also safely assume the new pages will be indexed soon since the spiders regularly visit the site.

You can also do things with custom 404 pages and .htaccess to point anyone linking to the old page to the new one as well as cyanide pointed out.