An ISD is required to obtain registration as a deemed supplier of services [section 23 read with para 6 of Schedule V].
The threshold limit of registration is not applicable to ISD.
The registration of ISD under the existing regime (i.e. under Service Tax) would not be migrated in GST regime.
All the existing ISDs will be required to obtain fresh registrations under new regime in case they want to operate as an ISD.