diff --git a/monai/utils/deprecate_utils.py b/monai/utils/deprecate_utils.py
index 64e678e2bd6..a741524d501 100644
--- a/monai/utils/deprecate_utils.py
+++ b/monai/utils/deprecate_utils.py
@@ -277,7 +277,7 @@ def deprecated_arg_default(
         # smaller than `since`, do nothing
         return lambda obj: obj
     if since is None and replaced is None:
-        # raise a DeprecatedError directly
+        # no versions given: the default has already changed, so always warn that it changed
         is_replaced = True
         is_deprecated = True
     else:
@@ -291,7 +291,9 @@ def _decorator(func):
         msg_prefix = f" Current default value of argument `{name}={old_default}`"
 
         if is_replaced:
-            msg_infix = f"was changed in version {replaced} from `{name}={old_default}` to `{name}={new_default}`."
+            msg_prefix = f" Default value of argument `{name}`"
+            in_version = f" in version {replaced}" if replaced is not None else ""
+            msg_infix = f"was changed{in_version} from `{name}={old_default}` to `{name}={new_default}`."
         elif is_deprecated:
             msg_infix = f"has been deprecated since version {since}."
             if replaced is not None:
diff --git a/tests/utils/test_deprecated.py b/tests/utils/test_deprecated.py
index 3171a67e2c3..9645d3df8ed 100644
--- a/tests/utils/test_deprecated.py
+++ b/tests/utils/test_deprecated.py
@@ -361,6 +361,37 @@ def foo(a, b="a"):
 
         self.assertWarns(FutureWarning, lambda: foo("a"))
 
+    def test_deprecated_arg_default_replaced_message(self):
+        """
+        Test the message for a changed default, with and without a known version.
+        """
+
+        @deprecated_arg_default("b", old_default="a", new_default="b", version_val=self.test_version)
+        def foo(a, b="b"):
+            return a, b
+
+        with self.assertWarns(FutureWarning) as aw:
+            foo("a")
+        msg = str(aw.warning)
+        self.assertIn("Default value of argument `b` was changed from `b=a` to `b=b`.", msg)
+        self.assertNotIn("version None", msg)
+        self.assertNotIn("Current default value", msg)
+
+        @deprecated_arg_default(
+            "b",
+            old_default="a",
+            new_default="b",
+            since=self.prev_version,
+            replaced=self.prev_version,
+            version_val=self.test_version,
+        )
+        def bar(a, b="b"):
+            return a, b
+
+        with self.assertWarns(FutureWarning) as aw:
+            bar("a")
+        self.assertIn(f"was changed in version {self.prev_version} from `b=a` to `b=b`.", str(aw.warning))
+
     def test_deprecated_arg_default_warning_with_none_as_placeholder(self):
         """
         Test deprecated arg default, where the default is used.