How to change a filter handler of the view?
I always take the following approach: Copy the existing views_data code via php:

// Copy the existing definition.
$data['table']['field_special'] = $data['table']['field'];
// Force the real field, so views still consider it as a different field in SQL.
$data['table']['field_special']['real field'] = 'field';
// Set a special handler.
$data['table']['field_special']['field']['handler'] = 'foo_handler_field_...';


This allows you to easy reuse the custom handler if needed, but at the same time not conflict with existing views.

In contrast to the approach of berliner (which certainly works) this does not add any runtime code execution, which adds potential bugs. This approach just uses basically configuration.