Need help for MySQL Upgrade from RDS Aurora MySQL 5.7 (2.11.0) to MySQL 8.0 default version

The issues you're experiencing seem to be related to connectivity problems between your application and the Aurora MySQL database, rather than directly related to the upgrade process from MySQL 5.7 to 8.0. However, it's important to note that upgrading between major versions of MySQL requires careful planning and testing.

For the specific errors you're seeing:

1. The application log error suggests a problem with establishing or maintaining a connection to the database. This could be due to network issues, firewall settings, or misconfigurations in the connection string.

2. The MySQL errors indicate failed connection attempts, including bad handshakes and errors in communication packets. These could be caused by incompatibilities between the MySQL client library version in your .NET application and the MySQL server version, or by network-related issues.

To address these issues and proceed with your upgrade, consider the following steps:

1. Verify that your .NET MySQL connector is compatible with both MySQL 5.7 and 8.0. You may need to update to the latest version of the connector.

2. Check your application's connection string to ensure it's correctly configured for the Aurora MySQL endpoint.

3. Review network configurations, including security groups and firewall rules, to ensure proper connectivity between your application and the database.

4. Test the connection using a simple command-line tool or MySQL Workbench to isolate whether the issue is specific to your application or a general connectivity problem.

5. Before proceeding with the upgrade, run the pre-upgrade checks provided by Amazon RDS. These checks can identify potential compatibility issues that need to be addressed before the upgrade.

6. Consider using Amazon RDS Blue/Green Deployments for the upgrade process, as this method can minimize application downtime and provide a safer upgrade path.

7. After addressing connectivity issues, thoroughly test your application with both MySQL 5.7 and 8.0 compatible Aurora clusters to ensure compatibility and proper functionality.

Remember that upgrading between major versions of MySQL can uncover incompatibilities in SQL syntax, stored procedures, or application code. It's crucial to review the MySQL 8.0 documentation for changes that might affect your application and to conduct comprehensive testing before and after the upgrade.
Sources
Amazon Aurora MySQL version 2 (with MySQL 5.7 compatibility) to version 3 (with MySQL 8.0 compatibility) upgrade checklist, Part 1 | AWS Database Blog
Troubleshooting for Aurora MySQL in-place upgrade - Amazon Aurora