This example adds more content around the values shown in a lookup field. It changes the options like '2.1', '2.2' to 'Version 2.1 is updated', 'Version 2.2 is updated'.

add_filter( 'frm_filtered_lookup_options', 'change_lookup_options', 10, 2 );
function change_lookup_options( $options, $args ) {
  if ( $args['field']->id == 25 ) { // change 25 to the id of the field in the other form
    foreach ( $options as $k => $option ) {
      $options[ $k ] = 'Version ' . $option . ' is updated';
    }
  }
  return $options;
}

There may be times when you want to use a Lookup field mapped to a User ID field from another form. By default, the User ID is shown rather than the display name. You can show the display name and the user ID as options with this example.

add_filter( 'frm_filtered_lookup_options', 'change_lookup_options', 10, 2 );
function change_lookup_options( $options, $args ) {
  if ( $args['field']->id == 808) { // change 808 to the id of the user ID field from the other form
    foreach ( $options as $k => $option ) {
      $user = get_user_by('id', $option);
      $value = $user->display_name;
      $options[ $k ] = $value . ' - ID ' . $option;
    }
  }
  return $options;
}

Replace 808 with the User ID field in your other form (the one that the lookup field is mapped to).

This example will populate a lookup field with uploaded image path

add_filter( 'frm_filtered_lookup_options', 'change_lookup_options', 10, 2 );
function change_lookup_options( $options, $args ) {
    if ( $args['field']->id === '25' ) { // change 25 to the ID of the file upload field ID in the other form
        foreach ( $options as $k => $option ) {
            $options[ $k ] = wp_get_attachment_url( $option );
        }
    }

    return $options;
}