To preserve the option to downgrade after upgrading your database to Oracle 12c release 2 (12.2), install the patch for bug 20898997 in your source ORACLE_HOME.
As part of your preupgrade plan, check your release to ensure that you can downgrade from an Oracle Database 12c release 2 (12.2) upgrade. To ensure that you can carry out a downgrade, you must have patch 20898997 installed in source ORACLE_HOME.
If your source Oracle Database Oracle home is not patched with patch 20898997, then it does not affect the upgrade. However, if you decide to downgrade the database, then the downgrade can fail. You receive an error message similar to the following:
ORA-00600: internal error code, arguments: 
There is no workaround to continue the downgrade.
To ensure that you can downgrade your database, complete the following procedure:
$ORACLE_HOME/OPatch/opatch lsinventory -bugs_fixed |grep –i "20898997"
A successful result is similar to the following:
Sub-patch 20898997; "XMLTYPESUP: QCTOXSNLB SHOULD NOT CHECK AGAINST SNAPSHOT SIZE"
If you do not see that the patch is installed, then proceed to the next step.