Yes, if the people are not satisfied with the category they're in, they can complain. There is another committee that looks at whether or not they are in the correct category, to see if they have to change, to go up or down. It depends. It's possible. When they receive their category the first time, people probably don't want to be in that category and want to be posted in another category.
Going through the service categories, when you are in category 8, you can stay in service, but if you go down to category 7, you probably have to leave the service. Taking that into account, sometimes when you go down through the categories you will not able to stay in the service any more, because of disablement or something like that. People sometimes prefer to accept their category, stay there, and then ask for a job in the service, but if you go down in the categories, you'll probably have to leave the service.
But you can apply to have your category changed if you don't agree with it.