You can do it with Automator and the "Change Extension" script here.
(scroll down the page a bit and click on the "Change Extension" heading of the descriptive paragraph, the script will download).
Once the script is downloaded, open Automator from your Apps folder. In the Library column, click "Files and Folders." From the column to the right of that, click and drag "Get Specified Finder Items" into the area that says "Drag actions or files here to start your workflow." You'll see a pane appear. From the Finder, drag your folder of items to be renamed onto that pane.
Now, from the Automator File menu item, click "Import Actions", navigate to the action you downloaded, highlight, and click "Import." Go back to your Library column and click "Recently Added", the new action will be there. Drag that under your "Get Specified Finder Items" action over on the right.
Now you have the files selected and the change extension action ready to go. Just type in the extension you want the files to have in the "Change Extensions" action pane, and click the run button on the top right, and you're done. Just make sure you have only the files you want to change in the folder specified.