Use this procedure to downgrade Oracle Standalone Cluster to Oracle Grid Infrastructure 18c after a successful upgrade.

1. As grid user, delete the Oracle Grid Infrastructure 19c Management Database:
   %ORACLE_HOME%\bin\dbca -silent -deleteDatabase -sourceDB -MGMTDB
2. As root user, use the command syntax rootcrs.bat -downgrade from 19c Grid home to downgrade Oracle Grid Infrastructure on all nodes, in any sequence. For example:
   %ORACLE_HOME%\crs\install\rootcrs.bat -downgrade
   Run this command from a directory that has write permissions for the Oracle Grid Infrastructure installation user. You can run the downgrade script in parallel on all cluster nodes, but one.
3. As root user, downgrade the last node after you downgrade all other nodes:
   %ORACLE_HOME%\crs\install\rootcrs.bat -downgrade
4. As grid user, remove Oracle Grid Infrastructure 19c Grid home as the active Oracle Clusterware home.

   Use the following command to start the installer, where C:\app\19.0.0\grid is the location of the new (upgraded) Grid home:

   Run the setup.exe command from the 19c Grid home. Add the flag -cfs if the Grid home is a shared home.

   cd %ORACLE_HOME%\oui\bin
   setup.exe -nowait -waitforcompletion -ignoreSysPrereqs
   -updateNodeList -silent CRS=false
   ORACLE_HOME=C:\app\19.0.0\grid
   "CLUSTER_NODES=node1,node2,node3" 

5. As grid user, set Oracle Grid Infrastructure 18c Grid home as the active Oracle Clusterware home:

   Use the following command to start the installer, where the path you provide for ORACLE_HOME is the location of the home directory from the earlier Oracle Clusterware installation:

   cd %ORACLE_HOME%\oui\bin 
   setup.exe -nowait -waitforcompletion -ignoreSysPrereqs 
   -updateNodeList -silent CRS=true 
   ORACLE_HOME=C:\app\18.0.0\grid
   "CLUSTER_NODES=node1,node2,node3"

6. As root user, start the 18c Oracle Clusterware stack on all nodes.
   crsctl start crs
7. As grid user, from any Oracle Grid Infrastructure 18c node, remove the MGMTDB resource as follows:
   %ORACLE_HOME%\bin\srvctl remove mgmtdb -f
8. As grid user, run DBCA in the silent mode from the 18c Grid home and create the Management Database container database (CDB) as follows:

   %ORACLE_HOME%\bin\dbca -silent -createDatabase -createAsContainerDatabase true 
   -templateName MGMTSeed_Database.dbc -sid -MGMTDB -gdbName _mgmtdb 
   -storageType ASM -diskGroupName ASM_DG_NAME
   -datafileJarLocation C:\app\18.0.0\grid\assistants\dbca\templates
   -characterset AL32UTF8 -autoGeneratePasswords -skipUserTemplateCheck

9. Configure the Management Database by running the Configuration Assistant from the location %ORACLE_HOME%\bin\mgmtca.bat —local.