When you change hosting companies, you will be asked to change nameservers too. This is to give the new hosting company complete control of your DNS. The new hosting company will give you the new nameservers, which should then be given to your registrar. There should always be a minimum of 2 nameservers given by the new hosting company.
Nameserver changes are free of charge and can be requested by submitting a support ticket through your control panel.
