What is the correct step after creating new servers when upgrading an existing forest?

Promoting at least two new servers to the role of Domain Controller (DC) is essential after creating new servers in the context of upgrading an existing Active Directory forest. This step ensures that there are multiple domain controllers available for redundancy and load balancing. When upgrading an Active Directory environment, having at least two DCs is critical for maintaining the health and connectivity of the directory services, as it allows for failover capabilities and ensures that directory services remain available even if one of the servers fails or is taken offline for maintenance.

Furthermore, by promoting new servers to DCs, you can gradually migrate roles and services from older servers, ensuring a smoother transition and reducing the risk of downtime or data loss during the upgrade process. This establishes a robust infrastructure that supports the continuity of operations as changes are made to the forest.

The other options, while they may be part of the overall upgrade strategy, do not directly address the immediate need for establishing redundancy in the domain controller role, which is why promoting new servers to DC is the most critical step at this stage.