An IP Domain that is used on a standalone Data Collector and already has Items is to be assigned to a Tenant-Agnostic Data Collector (TADCO) but fails with.
<Message>IP domain [ProxyIPDomainID] cannot be assigned to a Proxy IP Domain ProxyIPDomainID</Message>
This behavior is by design. It is because the IP domain already has devices discovered and if gets reassigned to a proxy IP domain we cannot guarantee the uniqueness of the existing device's IP. To move the devices, delete the devices and discover them after adding the IP Domain to the proxy DC.