To upgrade to RDS Oracle Database 21c (which is only CDB architecture), your database must use multitenant architecture. So conversion to CDB architecture is a prerequisite for major version upgrades to 21c and when you upgrade your database engine version, you cannot change the database architecture in the same operation. Therefore, you first need to convert the non-CDB to a CDB and then upgrade the 19c CDB to a 21c CDB in a separate step. Note: You can't convert a non-CDB to the multi-tenant configuration in a single modify-db-instance call. After you convert a non-CDB to a CDB, your CDB is in the single-tenant configuration. To convert the single-tenant configuration to the multi-tenant configuration, run modify-db-instance again.

Please follow the following steps :

-Step 1: Convert non-CDB to CDB (Single-tenant configuration): Select Oracle multitenant architecture in the Architecture settings.Upon conversion, your CDB will operate in a single-tenant configuration.For complete step by step procedure, please refer to the below document: [+] Converting an RDS for Oracle non-CDB to a CDB: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/oracle-cdb-converting.html

-Step 2: Convert single-tenant to multi-tenant configuration: In Architecture configuration, select Multi-tenant configuration.The existing single-tenant database will become the first tenant of the multi-tenant instance.For complete step by step procedure, please refer to the below document: [+] Converting the single-tenant configuration to multi-tenant: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/oracle-single-tenant-converting.html

-Step 3: Upgrade to 21c. After converting the architecture to Multi-tenant configuration, try upgrading the RDS engine version.

Important Considerations:

1. A new custom parameter group specific to the CDB with the particular database engine version must be created.
2. A new custom option group for CDB is required.
3. The conversion process is unidirectional - you can only convert from non-CDB to CDB, not vice versa. The conversion occurs immediately and cannot be scheduled for a maintenance window.
4. The upgrade process is irreversible.
5. The upgrade must be to a Release Update (RU) that was released in the same month or later.

Before You Upgrade: -Thoroughly review the RDS documentation and Oracle's upgrade guides for version-specific compatibility issues. This is particularly crucial for major version upgrades. -For database optimization, review Oracle best practices, such as gathering dictionary statistics. This ensures optimal performance during and after the upgrade. -you'll need to pre-create compatible parameter and option groups. This is a mandatory step that's often overlooked. -A critical step is validating all database objects and ensuring connectivity. Check that all databases links to external databases are functioning and accessible.

• For the storage ensure you have sufficient space for the upgrade process. Oracle upgrades can require significant temporary space. -Always take a manual snapshot before proceeding. While RDS creates automatic snapshots during upgrades, having your own backup provides additional security.

After You Upgrade: -you need to check for any invalid objects and recompile them as needed. -Review Oracle's documentation for version-specific post-upgrade steps. Each version may have unique requirements. -Thoroughly test your application functionality. -If issues arise, you can rollback using Point-in-Time Restore and a new DB instance will be created. -Finally, consider upgrading any database snapshots of deprecated versions to ensure long-term maintainability.

For CDB limitations, please refer to the following documents: [+] Limitations of RDS for Oracle CDBs: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Oracle.Concepts.CDBs.html#Oracle.Concepts.single-tenant-limitations

[+] Get started with the Multi-Tenant feature of Amazon RDS for Oracle:https://aws.amazon.com/blogs/database/get-started-with-the-multi-tenant-feature-of-amazon-rds-for-oracle/

The major version upgrade from 19c to 21c will incur downtime and the exact downtime duration will depend on factors like Database size, System resources, Storage performance, Database activity, Instance class etc…

Also we strong recommend that you test the upgrade before executing it in production environments and schedule these changes during off-business hours as they will result in downtime.