Override user_login in WordPress

override_user_login.php

// Check if user_login already exists before we force update
if ( ! username_exists( $username ) ) {

	// Force update user_login and user_nicename
	$tablename = $wpdb->prefix . "users";
	$wpdb->update( $tablename, 						// Table to Update 	( prefix_users )
				   array( 
				   		'user_login' 	=> $username,	// Data to Update 	( user_login )
				   		'user_nicename' => $username 	// Data to Update 	( user_nicename )
				   ),									
				   array( 'ID' => $user->ID ),			// WHERE clause 	( ID = $user->ID )
				   array(
				   		'%s',				// Data format 		( string )
				   		'%s'				// Data format 		( string )
				   	), 							
				   array('%d') 					// Where Format 	( int )
				 );
}