Review MOS Note: 2312911.1 - Before upgrading an Oracle Database from 12.1 to 12.2 or later

If you are planning to upgrade an Oracle Database from 12.1 to 12.2 or later, depending on your configuration, there is a possibility that the upgrade can result in immediate execution plan changes for a lot of SQLs due to the changes in adaptive feature in 12cR2.

Review the MOS Note: 2312911.1 for optimizer behavior (adaptive feature) change in 12cR2.