On the main node you can re-initialize the cluster if no standby nodes have been added. You can check if any standby nodes are in the cluster with:

opc cluster node list
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The screenshot shows that no other nodes are in the cluster, you can now remove the main node cluster config with:

opc cluster node reset
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The cluster config will identify that this is a main node with no standby nodes in the cluster. You will need to confirm that you want to proceed with removing the configuration

Now the node is no longer in a cluster you can re-initialize the cluster with your desired configuration.