Laravel Illuminate Schema : add method to updateColumn

gistfile1.php

<?php

## Migration ##
    Schema::table('users', function(Blueprint $table)
	{
		$table->updateColumn('username', 'string')->length(250)->nullable();
	});

## \Illuminate\Support\Database\Schema\Blueprint.php ##
    /**
     * Indicate that the given columns should be altered.
     *
     * @param  string  $column_name
     * @param  string  $type
     * @return \Illuminate\Support\Fluent
     */
	public function updateColumn($column_name, $type)
	{
		return $this->addCommand('updateColumn', compact('column_name', 'type'));
	}
	
## \Illuminate\Database\Schema\Grammars\MySqlGrammar.php ##
    /**
	 * Compile an update column command.
	 *
	 * @param  \Illuminate\Database\Schema\Blueprint  $blueprint
	 * @param  \Illuminate\Support\Fluent  $command
	 * @return string
	 */
	public function compileUpdateColumn(Blueprint $blueprint, Fluent $command)
	{
		$table = $this->wrapTable($blueprint);

		$type = $this->{'type'.ucfirst($command->type)}($command);

		return "alter table {$table} modify {$command->columnName} {$type}";
	}

I am facing issue it, code is not working for me. migration is successfully run but not changing the datatype.

return $this->addModifiers($sql, $blueprint, $command); // where is addmodifier function??