diff --git a/masque/library.py b/masque/library.py
index df637af..db9dda5 100644
--- a/masque/library.py
+++ b/masque/library.py
@@ -538,6 +538,7 @@ class ILibraryView(Mapping[str, 'Pattern'], metaclass=ABCMeta):
                 raise LibraryError('visit_* functions returned a new `Pattern` object'
                                    ' but no top-level name was provided in `hierarchy`')
 
+            del cast('ILibrary', self)[name]
             cast('ILibrary', self)[name] = pattern
 
         return self
diff --git a/masque/test/test_library.py b/masque/test/test_library.py
index d035db6..56ee3d7 100644
--- a/masque/test/test_library.py
+++ b/masque/test/test_library.py
@@ -221,6 +221,54 @@ def test_library_rename() -> None:
     assert "old" not in lib["parent"].refs
 
 
+def test_library_dfs_can_replace_existing_patterns() -> None:
+    lib = Library()
+    child = Pattern()
+    lib["child"] = child
+    top = Pattern()
+    top.ref("child")
+    lib["top"] = top
+
+    replacement_top = Pattern(ports={"T": Port((1, 2), 0)})
+    replacement_child = Pattern(ports={"C": Port((3, 4), 0)})
+
+    def visit_after(pattern: Pattern, hierarchy: tuple[str | None, ...], **kwargs) -> Pattern:       # noqa: ARG001
+        if hierarchy[-1] == "child":
+            return replacement_child
+        if hierarchy[-1] == "top":
+            return replacement_top
+        return pattern
+
+    lib.dfs(lib["top"], visit_after=visit_after, hierarchy=("top",), transform=True)
+
+    assert lib["top"] is replacement_top
+    assert lib["child"] is replacement_child
+
+
+def test_lazy_library_dfs_can_replace_existing_patterns() -> None:
+    lib = LazyLibrary()
+    lib["child"] = lambda: Pattern()
+    lib["top"] = lambda: Pattern(refs={"child": []})
+
+    top = lib["top"]
+    top.ref("child")
+
+    replacement_top = Pattern(ports={"T": Port((1, 2), 0)})
+    replacement_child = Pattern(ports={"C": Port((3, 4), 0)})
+
+    def visit_after(pattern: Pattern, hierarchy: tuple[str | None, ...], **kwargs) -> Pattern:       # noqa: ARG001
+        if hierarchy[-1] == "child":
+            return replacement_child
+        if hierarchy[-1] == "top":
+            return replacement_top
+        return pattern
+
+    lib.dfs(top, visit_after=visit_after, hierarchy=("top",), transform=True)
+
+    assert lib["top"] is replacement_top
+    assert lib["child"] is replacement_child
+
+
 def test_library_add_no_duplicates_respects_mutate_other_false() -> None:
     src_pat = Pattern(ports={"A": Port((0, 0), 0)})
     lib = Library({"a": Pattern()})