A phased migration approach, where specific subsets of data (both active and historical customer data) are migrated in stages, balances efficiency and minimal disruption. This strategy allows continuous operation of both legacy and new systems during the transition phase, minimizing downtime and service interruptions. It also provides opportunities to address issues on a smaller scale before they affect the entire dataset.

Ensuring the security of sensitive customer data during migration requires a multi-layered approach. Data encryption both at rest and in transit, via SSL/TLS protocols, ensures the data is protected against unauthorized access during transfer. User permissions and field-level security within Salesforce restrict access to sensitive data. Data anonymization or pseudonymization techniques mask sensitive information, providing additional privacy. "D. All of the above, forming a multi-layered approach to data security during and after migration" offers comprehensive protection for sensitive data throughout the migration process.
Reference: Salesforce Data Security

Workflow Rules in Salesforce can automate repetitive case escalation processes effectively. By setting up rules based on specific criteria, these workflows automatically escalate cases to the right team or individual when predefined conditions are met. This feature helps streamline case management processes, reduces manual intervention, and ensures timely response to critical issues, thereby improving overall efficiency in handling customer cases.

The Case Volume Deflection Rate is the key performance indicator that would effectively measure the success of increasing online self-service adoption. This KPI calculates the percentage of potential cases that are deflected through self-service channels such as FAQs, knowledge base articles, and automated chatbots, indicating how well these tools are serving customers' needs without requiring direct agent involvement. It is an effective measure of self-service adoption and efficiency.

Reference: Salesforce Deployment Considerations