How do I upgrade an RDS instance's PostgreSQL version using TerraForm?

Question

My current RDS instance is running PostgreSQL version 9.6.3, but I'd like to upgrade it to version 10.1. However, when I set the new version number in my TerraForm configuration and set allow_major_version_upgrade to true, I run into the following error:

Error: Error applying plan:

1 error(s) occurred:

* aws_db_instance.db_instance: 1 error(s) occurred:

* aws_db_instance.db_instance: Error modifying DB Instance db-example: InvalidParameterCombination: Cannot find upgrade path from 9.6.3 to 10.1.
  status code: 400, request id: <id>

In fact, I get a similar error message when I set 9.6.4 as the new version number - which already is confusing, since auto_minor_version_upgrade is also set to true, yet the instance is still at 9.6.3.

As you can see, there's still a lot of RDS and PostgreSQL I have to come to grips with, so any background info that can help me understand the problem would be very welcome as well.

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Edit:

After setting just allow_major_version_upgrade to true without actually modifying the version resulted in this error:

* aws_db_instance.db_instance: Error modifying DB Instance db-example: InvalidParameterCombination: No modifications were requested

Answer 1

As @ydaetskcoR pointed out, you can only upgrade to PostgreSQL versions actually supported by RDS, which is 9.6.5 at the moment.

The upgrade from 9.6.3 to 9.6.5 as a result of auto_minor_version_upgrade will also not happen as soon as it is supported, as described in an email Amazon sent a while ago:

Beginning January 22 2018, if your database instances have the “Auto Minor Version Upgrade” option enabled, we will automatically upgrade your PostgreSQL database instances, using the same scheme mentioned above, during your maintenance window. If you do not wish to apply these upgrades at this point in time, please set the “Auto Minor Version Upgrade” option to "No" before January 20, 2018.