On the Dashboard on Mijn SprookjesCraft, you can change your own rank in the server. In the box labeled "My products", press the button "Switch rank" next to the rank you want to change to. If you are in the server, you might get kicked. Next time you connect to the server, you'll have the chosen rank.
It's only possible to change between ranks you have access to. Players with access to the rank Medewerker are able to change between Bezoeker and Medewerker. This function only works if you have another rank to change to, sorry people with Bezoeker!