You can now easily upgrade the engine version on your existing ElastiCache for Redis clusters to utilize Amazon ElastiCache enhancements,including increased usable memory, improved synchronization and more efficient failovers. To achieve the smoothest performance on ElastiCache for Redis, or we recommend you upgrade to the latest engine version that is compatible with Redis 2.8.24. The engine upgrade process is designed to earn a best effort to retain your existing data and requires Redis replication to succeed. For more details and best practices on upgrading your ElastiCache for Redis engine version please refer to the ElastiCache Documentation.
Source: amazon.com