You could specify the meta key in the user query so the results you’re getting back are only for users who have an existing field.
For reference WP_User_Query
custom field(user meta) params.
$args = array(
'role' => 'Subscriber',
'meta_query' => array(
array(
'key' => 'old_meta_key_name',
'compare' => 'EXISTS'
)
),
);
$users_with_meta = new WP_User_Query( $args );
$users = $users_with_meta->get_results();