As an administrator of the ISPConfig server it is possible to move sites from one client to another by using the Tools > Move site option. This nice option seems also to be available for resellers, as it is shown within the Tools menu, however it is not allowed to use it. (An access denied message occurs). Will the Tools > move site function also be available for resellers in the near future as well or not?
You will have to move the client to the new reseller and not the site as sites are not directly connected to resellers. You can change the reseller of a client by selecting another group at the top of the client form.