Is it detrimental to make a site wide change from .html to .shtml (all pages)?

[BVREID]

We have an established website with decent domain authority.  My developer inherited the site from another developer and is recommending that we convert all pages from the .html to the .shmtl  From an SEO perspective, would this hurt us?

Also, if this is not an issue, would updating the canonical help us, or does the canonical setting only deal with the "www." vs. "non-www"?

Any insights will be appreciated greatly.  Thanks!

[MattAntonino]

If you have decent rankings already I would hesitate to do this. While your rankings will eventually come back to where they're at now, there will DEFINITELY be some time in the middle where you're not doing as well.  The 301s suggested in other answers are a great (and necessary) idea.

If you don't HAVE to do this, I would question it as well. It will definitely affect your site.

[Martijn_Scheijbeler]

Hi Barry,

Well from a marketing perspective I would always go back to your developer and say that it's not an option to change the URL structure only because of technical dependencies. You build up your reputation and with changing the URLs, even if you redirect them to the new location there always will be a certain loss in authority.

[Matt-Williamson]

If you decide to go down this route the best way to minimise SEO impact would be to implement 301 redirects from the .html to the new .shtml.

Have a look at the Redirect While Changing File Extensions section on Moz's Guide to redirects here - http://moz.com/learn/seo/redirection

This page is well worth a read in order for you to get a better understanding but I thought I would draw your attention to this section to help put your mind at ease -

"There are multiple options for doing this, but in general, the 301 redirect is preferable for both users and search engines. Serving a 301 indicates to both browsers and search engine bots that the page has moved permanently. Search engines interpret this to mean that not only has the page changed location, but that the content—or an updated version of it—can be found at the new URL. The engines will carry any link weighting from the original page to the new URL"

In terms of what loss of value a 301 redirect will have no one is sure and what impact this will have. 301 redirects would be the best way to do what is being suggested by your developer with minimal seo impact. I think you might find this post interesting - http://moz.com/community/q/how-much-juice-do-you-lose-in-a-301-redirect

Hope this helps