Reserved DB Instances:

“You can now make a one-time, up-front payment to create a one or three year reservation to run your DB Instance in a specific Region and receive a significant discount off of the ongoing hourly usage charge.” –Amazon
See: Announcing DB Engine Version Management, MySQL 5.1.49 with InnoDB Plugin

DB Engine Version Management:

“Now, DB Engine Version Management gives you optional control over which Amazon RDS supported MySQL version powers your DB Instance and support for MySQL 5.1.49 with InnoDB Plugin.” –Amazon
See: Amazon RDS adds new Reserved Database Instance option