When attempting an upgrade, I received the following error:

Error: updating RDS Cluster (retool-temporal-temporal-rds-instance): operation error RDS: ModifyDBCluster, https response error StatusCode: 400, RequestID: cc692cbf-df40-4bec-a74a-f1bbe6f7b2f1, api error InvalidParameterCombination: The AllowMajorVersionUpgrade flag must be present when upgrading to a new major version.

Looking at the Terraform module, it seems that the most relevant location is here. There's no way to pass that flag to the module, so there's no possible way to upgrade it from the Terraform.

The same issue is present here.

Proposed solution

Create a variable called allow_major_version_upgrade in the retool module with a default value of false, and pass that to both aws_db_instance.this and the module above.

Update the user documentation so that it's clear when this is necessary (probably needs to be part of release notes? Or at least a troubleshooting section so a Google search will come up with the instructions)