You want to update your existing VPC Service Controls perimeter with a new access level.

Enable Dry Run Mode: Start by enabling the dry run mode for your VPC Service Controls perimeter. This mode allows you to test changes without actually enforcing them, thus preventing any disruption to your current setup.

Add Access Level: Add your new access level to the dry run configuration. This way, you can monitor how the new access level would behave and interact with your existing setup without any real impact.

Vetting Process: Carefully vet the new access level by analyzing logs and monitoring the behavior in the dry run mode. Ensure that the new configuration meets your security and operational requirements.

Update Perimeter: Once you are confident that the new access level will not disrupt existing services and meets all requirements, update the actual perimeter configuration with the new access level. This approach minimizes risk by allowing you to test changes before they take effect, ensuring seamless updates with minimal disruption. References:

Google Cloud - Configuring VPC Service Controls

Google Cloud - Using Dry Run Mode