Unable to create Blue/Green deployment for same engine version of Aurora Postgres

Based on the information available, it appears that you are encountering an unexpected issue when trying to create a Blue/Green deployment for Aurora PostgreSQL with the same engine version. The error message "Cannot find upgrade path from 16.4 to 16.4" suggests that the system is expecting a version upgrade, even though this is not explicitly stated as a requirement in the documentation.

However, it's important to note that Blue/Green deployments are typically used for making changes to the database environment, including version upgrades. While the documentation doesn't explicitly state that you must upgrade the engine version, it does mention that you can specify a higher DB engine version when creating a Blue/Green deployment.

That being said, Blue/Green deployments should theoretically support creating a staging environment with the same engine version, as they are also used for other types of changes such as modifying database parameters or making schema changes.

Given that you're encountering this error, it's possible that there might be a current limitation or bug in the Aurora PostgreSQL Blue/Green deployment process that requires a version upgrade. This isn't explicitly documented, which explains why you couldn't find this information in the official documentation.

To resolve this issue, you might consider the following options:

1. Try creating the Blue/Green deployment with a minor version upgrade (if available).
2. Contact AWS Support to report this issue and get clarification on whether this is expected behavior or a bug.
3. If you don't need to make engine version changes, consider alternative methods for making your desired changes, such as creating a clone of your DB cluster and applying changes there before switching over.

It's worth noting that AWS regularly updates its services, so this behavior might change in the future. Always check the most recent documentation and consider reaching out to AWS Support for the most up-to-date information on service capabilities and limitations.
Sources
Overview of Amazon Aurora Blue/Green Deployments - Amazon Aurora
Limitations and considerations for blue/green deployments - Amazon Aurora