public function passive_migration() {
		$this->doing_passive_migration = true;

		for ( $i = $this->data_version; $this->data_version < $this->model_version; $i ++ ) {
			// Process migration for current version. ex. current version is 2, runs pum_theme_passive_migration_2.
			do_action_ref_array( 'pum_theme_passive_migration_' . $this->data_version, array( &$this ) );
			$this->data_version ++;

			/**
			 * Update the themes data version.
			 */
			$this->update_meta( 'popup_theme_data_version', $this->data_version );
		}

		do_action_ref_array( 'pum_theme_passive_migration', array( &$this, $this->data_version ) );

		$this->doing_passive_migration = false;
	}