If you want to safely change the name of a custom post type in your WordPress site? Follow the below steps:
1 Download and use the plugin : http://wordpress.org/extend/plugins/ptypeconverter/
2 Copy your custom post type “portfolio” file somewhere save. call it for example portfolio_post_typeBACKUP.php
3 Now you are sure when this method fails. you can recover it.
4 Change “portfolio” into “projects”
5 Import the posts with the plugin and viola!
Hope this works.
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