convert_form_settings_to_action.php

<?php

add_filter( 'ninja_forms_upgrade_settings', 'convert_form_settings_to_action' );
function convert_form_settings_to_action( $form_data ){

  if( isset( $data[ 'settings' ][ 'my_processing_flag' ] ) && 1 == $data[ 'settings' ][ 'my_processing_flag' ] ){
  
    $new_action = array(
        'type' => 'action_type',
        'label' => __( 'My Action', 'ninja-forms-paypal-express' ),
        'new_setting' => $data[ 'settings' ][ 'old_plugin_setting' ]
    );
    
    $form_data[ 'actions' ][] = $new_action;
  }
  
  return $form_data;
}