Changing old [display_podcast] shortcode to new [powerpress] shortcode

You had PodPress previously correct? If that is the case, then go to PowerPress Settings > Basic Settings, near the bottom there is a setting to process PodPress episodes. enable that, then click save. PowerPress will then process the [display_podcast] shortcode.

Otherwise, you will need to us a search replace plugin to replace [display_podcast] with [powerpress]. Remember to backup your database before using such a plugin.