There are several options to upgrade an Autonomous Database instance to Oracle Database 23ai. You can upgrade with scheduled upgrade or you can upgrade by cloning.

Using scheduled upgrade, an instance is upgraded to Oracle Database 23ai automatically according to the schedule you specify.

Using cloning to upgrade, when you clone an Autonomous Database instance and the source database version is Oracle Database 19c, you can choose Oracle Database 23ai for the clone.

• Upgrade to Oracle Database 23ai with Scheduled Upgrade
  If your Autonomous Database instance is using Oracle Database 19c, you can upgrade to Oracle Database 23ai with Scheduled Upgrade.
• Upgrade to Oracle Database 23ai by Creating a Full Clone
  If your Autonomous Database instance is using Oracle Database 19c you can upgrade to Oracle Database 23ai by creating a full clone.
• Upgrade to Oracle Database 23ai by Creating a Refreshable Clone
  You can create a refreshable clone where the source database version is Oracle Database 19c and the cloned database version is Oracle Database 23ai.