.htaccess Redirection

Looking for guidance for Proper Redirect

[Eric_Lander]

I've got a site that has pages indexed in the major engines, but I need to reformat the URL structures. I now need to figure out how to forward the traffic in order not to disrupt the rankings, but also create a URL structure that contains a proper file extension.

Currently, the URL's indexed are in the format of:
...domain.com/product/blue-widget

You'll see that there is no trailing slash or file extension. I want to change these URLs over to the format of:

...domain.com/product/blue-widget.php

What adjustments would I need to make to my htaccess file? Currently, the syntax is:

RewriteRule ^product/(.*)$ /product.php?ID=$1 [L]

Any advice or insight would be greatly appreciated.