Final answer:
ElastiCache for Redis may automatically replace a primary node in cases of failure, and might also do so during scaling operations or software updates.
Step-by-step explanation:
Whether ElastiCache for Redis needs to replace a primary node is typically determined by the circumstances surrounding the need for node replacement such as node failure, scaling operations, or software updates. AWS ElastiCache for Redis is designed to manage the failover process automatically in the event of a node failure. If a primary node fails, ElastiCache automatically promotes a replica to primary status. However, for scaling operations, you may choose to replace nodes to adjust the capacity. Similarly, during software updates, the service may replace nodes to ensure that the latest software version is running.