Will a change in my url suffix affect my URL rankings?

I am trying to carry a legacy website to a new CMS without hurting its SEO rankings. My old legacy urls are of type:

www.example.com/page-url.html

And my new urls look like

www.example.com/page-url

The problem is that the SEO agency I work with says that these are two different pages and google will see them as unique.
Is this true? What can I do?

As long as you redirect the old URL, so that anyone going to example.com/page-url.html is taken to example.com/page-url, you won’t have a problem – you might see a slight temporary dip while Google sorts its index out but nothing more than that.

Hey Stevie,

Thanks for replying. The SEO agency says 2 to 3 months worth of losses on rankings. Do you mean its gonna be smaller than that?
And I should use 3.0 permanent redirect correct?

I am trying to follow this article: http://www.seomoz.org/learn-seo/redirection
Do you recommend any others that are authoritative?

If you use a permanent redirect then the search engines will update their links to go to the new location instead of the old one. Also since the existing entries in the search results still point to pages with the same content even though the address has changed there is no reason why they should make any changes to where those entries are in the results. You should only get a dip if you don’t use a permanent redirect.

I would be surprised if it is as much as that given that your site is staying on the same domain, I suspect they are just covering their backs in case you get a worst case scenario, but I’m not giving any promises!

And I should use 3.0 permanent redirect correct?

Nearly, it’s a 301 permanent redirect.

I am trying to follow this article: http://www.seomoz.org/learn-seo/redirection
Do you recommend any others that are authoritative?

That all looks pretty accurate to me. The next thing to check is whether the CMS you use gives you the option to use RewriteEngine in .htaccess, or if it has a rewrite module of its own.