I cannot find any information in the AWS documentation that modifying the Redis engine version will or will not cause downtime. It does not explain how the upgrade occurs other than it's performed in the maintenance window.
It is safe to upgrade a production Elasticache Redis instance via the AWS console without loss of data or downtime?
Note: The client library we use is compatible with all versions of Redis so the application should not notice the upgrade.
Changing a cache engine version is a disruptive process which clears all cache data in the cluster. **
I don't now the requirements of your particular application. But if you can't lose your data and you need to do a major version upgrade, it would be advisable to migrate to a new cluster rather than upgrading the current setup.
** http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/VersionManagement.html