How to Override Views Field Handler
You don't need to create a new field handler. You can replace the class of the existing field plugin. The plugin manager provides a hook to alter the plugin definitions:

/**
 * Implements hook_views_plugins_field_alter().
 */
function mymodule_views_plugins_field_alter(array &amp;$plugins) {
  $plugins['geolocation_field_proximity']['class'] = 'Drupal\mymodule\Plugin\views\field\RangeProximityField';
}