yeah we recently migrated from memcached on elasticache to redis on EC2 (and bought the reservations), though our migration rolled out about 4-6 weeks ago. oh well, I guess.
"Seamless Integration: If you are running Redis on EC2, you can transfer its contents to a new Amazon ElastiCache for Redis node. You may also attach a Redis node running on EC2 to an Amazon ElastiCache for Redis node."[1]
... at least after your reservations run out.
[1] https://aws.amazon.com/about-aws/whats-new/2013/09/04/amazon...