diff --git a/spec/Node/SimpleNodeSpec.php b/spec/Node/SimpleNodeSpec.php
index 0b5357b..28dcd67 100644
--- a/spec/Node/SimpleNodeSpec.php
+++ b/spec/Node/SimpleNodeSpec.php
@@ -4,7 +4,10 @@
 
 namespace spec\chrisjenkinson\StructuredDocumentParser\Node;
 
+use chrisjenkinson\StructuredDocumentParser\Node\DuplicateNodeException;
 use chrisjenkinson\StructuredDocumentParser\Node\NodeInterface;
+use chrisjenkinson\StructuredDocumentParser\Node\NodeNotFoundException;
+use chrisjenkinson\StructuredDocumentParser\Node\SimpleNode;
 use PhpSpec\ObjectBehavior;
 use RuntimeException;
 
@@ -40,24 +43,90 @@ public function it_has_nodes(NodeInterface $node): void
 
     public function it_throws_exception_if_no_such_child_exists(): void
     {
-        $this->shouldThrow(RuntimeException::class)->duringGetNode('nonexistent');
+        $this->shouldThrow(NodeNotFoundException::class)->duringGetNode('nonexistent');
     }
 
-    public function it_exports_a_tree_as_a_string(NodeInterface $node): void
+    public function it_throws_when_adding_a_node_whose_name_is_already_taken(NodeInterface $node, NodeInterface $other): void
     {
         $node->getName()->willReturn('ChildNode');
-        $node->__toString()->willReturn('');
+        $other->getName()->willReturn('ChildNode');
 
-        $this->setAttribute('something', 'result');
         $this->addNode($node);
 
+        $this->shouldThrow(DuplicateNodeException::class)->duringAddNode($other);
+        $this->getNode('ChildNode')->shouldReturn($node);
+    }
+
+    public function it_replaces_a_node_with_the_same_name(NodeInterface $node, NodeInterface $replacement): void
+    {
+        $node->getName()->willReturn('ChildNode');
+        $replacement->getName()->willReturn('ChildNode');
+
+        $this->addNode($node);
+        $this->replaceNode($replacement);
+
+        $this->getNodes()->shouldReturn(['ChildNode' => $replacement]);
+    }
+
+    public function it_throws_when_replacing_a_node_that_does_not_exist(NodeInterface $node): void
+    {
+        $node->getName()->willReturn('ChildNode');
+
+        $this->shouldThrow(NodeNotFoundException::class)->duringReplaceNode($node);
+    }
+
+    public function it_exports_a_tree_as_a_string(): void
+    {
+        $child      = new SimpleNode();
+        $grandchild = new SimpleNode();
+        $listItem   = new SimpleNode();
+
+        $grandchild->setAttribute('depth', 2);
+        $listItem->setAttribute('item', 'first');
+        $child->addNode($grandchild);
+        $child->setAttribute('items', [$listItem]);
+
+        $this->setAttribute('something', 'result');
+        $this->addNode($child);
+
         $this->__toString()->shouldReturn('{
     "attributes": {
         "something": "result"
     },
     "nodes": {
-        "ChildNode": {}
+        "SimpleNode": {
+            "attributes": {
+                "items": [
+                    {
+                        "attributes": {
+                            "item": "first"
+                        },
+                        "nodes": []
+                    }
+                ]
+            },
+            "nodes": {
+                "SimpleNode": {
+                    "attributes": {
+                        "depth": 2
+                    },
+                    "nodes": []
+                }
+            }
+        }
+    }
+}');
     }
+
+    public function it_replaces_invalid_utf8_when_exporting_as_a_string(): void
+    {
+        $this->setAttribute('bad', "a\xffb");
+
+        $this->__toString()->shouldReturn('{
+    "attributes": {
+        "bad": "a\ufffdb"
+    },
+    "nodes": []
 }');
     }
 }
diff --git a/spec/NodeTraverser/NodeTraverserSpec.php b/spec/NodeTraverser/NodeTraverserSpec.php
index 6033219..59daa6d 100644
--- a/spec/NodeTraverser/NodeTraverserSpec.php
+++ b/spec/NodeTraverser/NodeTraverserSpec.php
@@ -28,15 +28,17 @@ public function it_can_traverse_a_node(NodeInterface $node, NodeVisitorInterface
         $nodeVisitor->afterTraverse($node)->shouldHaveBeenCalled();
     }
 
-    public function it_can_add_a_child(NodeInterface $node, NodeInterface $child, NodeVisitorInterface $nodeVisitor, NodeVisitorInterface $nodeVisitor2): void
+    public function it_puts_a_traversed_child_back_in_place(NodeInterface $node, NodeInterface $child, NodeVisitorInterface $nodeVisitor, NodeVisitorInterface $nodeVisitor2): void
     {
         $node->getNodes()->willReturn([$child]);
         $node->getAttributes()->willReturn([]);
 
         $child->getNodes()->willReturn([]);
         $child->getAttributes()->willReturn([]);
+        $child->getName()->willReturn('ChildNode');
 
-        $node->addNode($child)->shouldBeCalled();
+        $node->replaceNode($child)->shouldBeCalled();
+        $node->addNode($child)->shouldNotBeCalled();
 
         $this->addVisitor($nodeVisitor);
         $this->addVisitor($nodeVisitor2);
diff --git a/src/Node/AbstractNode.php b/src/Node/AbstractNode.php
index 8558774..e3f70f6 100644
--- a/src/Node/AbstractNode.php
+++ b/src/Node/AbstractNode.php
@@ -21,7 +21,15 @@ abstract class AbstractNode implements NodeInterface
 
     public function __toString(): string
     {
-        return json_encode(['attributes' => $this->attributes, 'nodes' => $this->nodes], JSON_PRETTY_PRINT);
+        return json_encode($this, JSON_PRETTY_PRINT | JSON_INVALID_UTF8_SUBSTITUTE | JSON_THROW_ON_ERROR);
+    }
+
+    /**
+     * @return array{attributes: array, nodes: NodeInterface[]}
+     */
+    public function jsonSerialize(): array
+    {
+        return ['attributes' => $this->attributes, 'nodes' => $this->nodes];
     }
 
     /**
@@ -49,7 +57,7 @@ public function getNode(string $key): NodeInterface
             return $this->nodes[$key];
         }
 
-        throw new RuntimeException(sprintf('No such node "%s"', $key));
+        throw new NodeNotFoundException($key);
     }
 
     /**
@@ -62,6 +70,19 @@ public function getNodes(): array
 
     public function addNode(NodeInterface $node): void
     {
+        if ($this->hasNode($node->getName())) {
+            throw new DuplicateNodeException($node->getName());
+        }
+
+        $this->nodes[$node->getName()] = $node;
+    }
+
+    public function replaceNode(NodeInterface $node): void
+    {
+        if (!$this->hasNode($node->getName())) {
+            throw new NodeNotFoundException($node->getName());
+        }
+
         $this->nodes[$node->getName()] = $node;
     }
 
diff --git a/src/Node/DuplicateNodeException.php b/src/Node/DuplicateNodeException.php
new file mode 100644
index 0000000..e783bb7
--- /dev/null
+++ b/src/Node/DuplicateNodeException.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace chrisjenkinson\StructuredDocumentParser\Node;
+
+use LogicException;
+
+class DuplicateNodeException extends LogicException
+{
+    public function __construct(string $name)
+    {
+        parent::__construct(sprintf('A node named "%s" already exists', $name));
+    }
+}
diff --git a/src/Node/NodeInterface.php b/src/Node/NodeInterface.php
index 65c28ce..4898950 100644
--- a/src/Node/NodeInterface.php
+++ b/src/Node/NodeInterface.php
@@ -4,7 +4,9 @@
 
 namespace chrisjenkinson\StructuredDocumentParser\Node;
 
-interface NodeInterface
+use JsonSerializable;
+
+interface NodeInterface extends JsonSerializable
 {
     public function __toString(): string;
 
@@ -35,5 +37,7 @@ public function getNodes(): array;
 
     public function addNode(NodeInterface $node): void;
 
+    public function replaceNode(NodeInterface $node): void;
+
     public function removeNode(NodeInterface $node);
 }
diff --git a/src/Node/NodeNotFoundException.php b/src/Node/NodeNotFoundException.php
new file mode 100644
index 0000000..6df07eb
--- /dev/null
+++ b/src/Node/NodeNotFoundException.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace chrisjenkinson\StructuredDocumentParser\Node;
+
+use RuntimeException;
+
+class NodeNotFoundException extends RuntimeException
+{
+    public function __construct(string $name)
+    {
+        parent::__construct(sprintf('No such node "%s"', $name));
+    }
+}
diff --git a/src/NodeTraverser/NodeTraverser.php b/src/NodeTraverser/NodeTraverser.php
index dcef299..dc91eaa 100644
--- a/src/NodeTraverser/NodeTraverser.php
+++ b/src/NodeTraverser/NodeTraverser.php
@@ -121,10 +121,13 @@ private function runTraverseNodeOnSubNodes(NodeInterface $node, array $children)
                 return;
             }
 
-            if (get_class($newChild) !== get_class($child)) {
-                $node->removeNode($child);
+            if ($newChild->getName() === $child->getName()) {
+                $node->replaceNode($newChild);
+
+                return;
             }
 
+            $node->removeNode($child);
             $node->addNode($newChild);
         }, $children);
     }