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A year or so ago, they introduced the ability to change the instance type between the same family type [1]. [1] http://docs.aws.amazon.com/AWSEC2/latest/UserGu

• _delirium 12y ago
  That increases the flexibility, but afaict doesn't help with the upgradeability angle. When Amazon passes along the technological dividend by creating new instances with a better price/performance profile, it's usually as new family types. So your reserved t2.* instance can swap with other t2.* instances, but won't be able to take advantage of any t3.* instance that's introduced in the future. It's not a huge deal, just something to take into account when projecting out costs: the 3-year reserved instances are locking in today's prices until 2017, in return for a discount over today's non-reserved prices. Whether this produces long term gains requires some assumptions about how the market will change over the next 3 years.