diff --git a/command/format/diff_test.go b/command/format/diff_test.go
index 5fc4637dc..4a1d7111c 100644
--- a/command/format/diff_test.go
+++ b/command/format/diff_test.go
@@ -1002,6 +1002,80 @@ func TestResourceChange_primitiveSet(t *testing.T) {
       ~ id        = "i-02ae66f368e8518a9" -> (known after apply)
       + set_field = []
     }
+`,
+		},
+		"in-place update to unknown": {
+			Action: plans.Update,
+			Mode:   addrs.ManagedResourceMode,
+			Before: cty.ObjectVal(map[string]cty.Value{
+				"id":  cty.StringVal("i-02ae66f368e8518a9"),
+				"ami": cty.StringVal("ami-STATIC"),
+				"set_field": cty.SetVal([]cty.Value{
+					cty.StringVal("aaaa"),
+					cty.StringVal("bbbb"),
+				}),
+			}),
+			After: cty.ObjectVal(map[string]cty.Value{
+				"id":        cty.UnknownVal(cty.String),
+				"ami":       cty.StringVal("ami-STATIC"),
+				"set_field": cty.UnknownVal(cty.Set(cty.String)),
+			}),
+			Schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"id":        {Type: cty.String, Optional: true, Computed: true},
+					"ami":       {Type: cty.String, Optional: true},
+					"set_field": {Type: cty.Set(cty.String), Optional: true},
+				},
+			},
+			RequiredReplace: cty.NewPathSet(),
+			ExpectedOutput: `  # test_instance.example will be updated in-place
+  ~ resource "test_instance" "example" {
+        ami       = "ami-STATIC"
+      ~ id        = "i-02ae66f368e8518a9" -> (known after apply)
+      ~ set_field = [
+          - "aaaa",
+          - "bbbb",
+        ] -> (known after apply)
+    }
+`,
+		},
+		"in-place update to unknown element": {
+			Action: plans.Update,
+			Mode:   addrs.ManagedResourceMode,
+			Before: cty.ObjectVal(map[string]cty.Value{
+				"id":  cty.StringVal("i-02ae66f368e8518a9"),
+				"ami": cty.StringVal("ami-STATIC"),
+				"set_field": cty.SetVal([]cty.Value{
+					cty.StringVal("aaaa"),
+					cty.StringVal("bbbb"),
+				}),
+			}),
+			After: cty.ObjectVal(map[string]cty.Value{
+				"id":  cty.UnknownVal(cty.String),
+				"ami": cty.StringVal("ami-STATIC"),
+				"set_field": cty.SetVal([]cty.Value{
+					cty.StringVal("aaaa"),
+					cty.UnknownVal(cty.String),
+				}),
+			}),
+			Schema: &configschema.Block{
+				Attributes: map[string]*configschema.Attribute{
+					"id":        {Type: cty.String, Optional: true, Computed: true},
+					"ami":       {Type: cty.String, Optional: true},
+					"set_field": {Type: cty.Set(cty.String), Optional: true},
+				},
+			},
+			RequiredReplace: cty.NewPathSet(),
+			ExpectedOutput: `  # test_instance.example will be updated in-place
+  ~ resource "test_instance" "example" {
+        ami       = "ami-STATIC"
+      ~ id        = "i-02ae66f368e8518a9" -> (known after apply)
+      ~ set_field = [
+            "aaaa",
+          - "bbbb",
+          ~ (known after apply),
+        ]
+    }
 `,
 		},
 	}