I was attempting to customize values of some of the existing well-known nodes in the predefined address space.

Others UA implementations offer "re-binding" solution where you just re-map a node-id with your callbacks and/or data.

Is there an easy way, with existing sample code, to easily re-bind a predefined node ? (Could only find documentation about creating own custom address space, not modifying existing nodes)

I was a bit puzzled by the classes CoreNodeManager and ConfigurationNodeManager, with their different public interfaces, and came up with a solution. But it looks way too complicated !

To add a node,  linked to existing system structure :

// Create a new NodeState :
var myNewNodeState = new BaseDataVariableState<string>(existingparent) //or new PropertyState()
// Link it with parent : 
existingparent.AddChild(myNewNodeState)
// Import the node into the CoreNodeManager and AddReference within the CoreNodeManager 
server.CoreNodeManager.ImportNodes(server.DefaultSystemContext, new NodeState[] { myNewNodeState });
server.CoreNodeManager.AddReference(existingparent.NodeId, ReferenceTypeIds.HasProperty, false, myNewNodeState.NodeId, false);

To replace a node in predefined system structure :

// Create a new NodeState :
var myNewNodeState = new BaseDataVariableState<string>(existingparent) //or new PropertyState()
// REPLACE it with parent : 
existingparent.ReplaceChild(myNewNodeState)

// HACK : Get ConfigurationNodeManager (my node is handled by this manager)
INodeManager myNodeMgr;
Opc.Ua.Server.NodeHandle objectX = (Opc.Ua.Server.NodeHandle)server.NodeManager.GetManagerHandle(myNewNodeState.NodeId, out myNodeMgr);
var cfgManager = myNodeMgr as ConfigurationNodeManager;
// DELETE THE NODE REFERENCE FROM ConfigManager AND CoreManager
cfgManager.DeleteNode(server.DefaultSystemContext, myNewNodeState.NodeId);
server.CoreNodeManager.DeleteNode(myNewNodeState.NodeId, deleteChildren: true, silent: true);

// Import the node into the CoreNodeManager and AddReference within the CoreNodeManager  (can't re-import in ConfigManager without screwing the NodeId!)
server.CoreNodeManager.ImportNodes(server.DefaultSystemContext, new NodeState[] { myNewNodeState });
server.CoreNodeManager.AddReference(existingparent.NodeId, ReferenceTypeIds.HasProperty, false, myNewNodeState.NodeId, false);

Maybe there is a less convoluted way to do this ?