update: 31 Oct 2018, Still usable on laravel 5.7 https://laravel.com/docs/5.7/migrations#modifying-columns
To make some change to existing db, you can modify column type by using change() in migration.
This is what you could do
Schema::table('orders', function ($table) {
$table->string('category_id')->change();
});
please note you need to add doctrine/dbal dependency to composer.json
for more information you can find it here http://laravel.com/docs/5.1/migrations#modifying-columns