diff --git a/config/hcl2shim/paths.go b/config/hcl2shim/paths.go
index 99437cbb1..3403c026b 100644
--- a/config/hcl2shim/paths.go
+++ b/config/hcl2shim/paths.go
@@ -251,3 +251,26 @@ func pathFromFlatmapKeySet(key string, ty cty.Type) (cty.Path, error) {
 	// set as a whole changed.
 	return nil, nil
 }
+
+// FlatmapKeyFromPath returns the flatmap equivalent of the given cty.Path for
+// use in generating legacy style diffs.
+func FlatmapKeyFromPath(path cty.Path) string {
+	var parts []string
+
+	for _, step := range path {
+		switch step := step.(type) {
+		case cty.GetAttrStep:
+			parts = append(parts, step.Name)
+		case cty.IndexStep:
+			switch ty := step.Key.Type(); {
+			case ty == cty.String:
+				parts = append(parts, step.Key.AsString())
+			case ty == cty.Number:
+				i, _ := step.Key.AsBigFloat().Int64()
+				parts = append(parts, strconv.Itoa(int(i)))
+			}
+		}
+	}
+
+	return strings.Join(parts, ".")
+}
diff --git a/config/hcl2shim/paths_test.go b/config/hcl2shim/paths_test.go
index cffbe6b5a..ab166e6f8 100644
--- a/config/hcl2shim/paths_test.go
+++ b/config/hcl2shim/paths_test.go
@@ -3,6 +3,7 @@ package hcl2shim
 import (
 	"fmt"
 	"reflect"
+	"strconv"
 	"strings"
 	"testing"
 
@@ -365,5 +366,43 @@ func TestRequiresReplace(t *testing.T) {
 		})
 
 	}
-
+}
+
+func TestFlatmapKeyFromPath(t *testing.T) {
+	for i, tc := range []struct {
+		path cty.Path
+		attr string
+	}{
+		{
+			path: cty.Path{
+				cty.GetAttrStep{Name: "force_new"},
+			},
+			attr: "force_new",
+		},
+		{
+			path: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.NumberIntVal(0)},
+				cty.GetAttrStep{Name: "force_new"},
+			},
+			attr: "attr.0.force_new",
+		},
+		{
+			path: cty.Path{
+				cty.GetAttrStep{Name: "attr"},
+				cty.IndexStep{Key: cty.StringVal("key")},
+				cty.GetAttrStep{Name: "obj_attr"},
+				cty.IndexStep{Key: cty.NumberIntVal(0)},
+				cty.GetAttrStep{Name: "force_new"},
+			},
+			attr: "attr.key.obj_attr.0.force_new",
+		},
+	} {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+			attr := FlatmapKeyFromPath(tc.path)
+			if attr != tc.attr {
+				t.Fatalf("expected:%q got:%q", tc.attr, attr)
+			}
+		})
+	}
 }
diff --git a/helper/resource/testing_config.go b/helper/resource/testing_config.go
index 7635a7057..311fdb6ef 100644
--- a/helper/resource/testing_config.go
+++ b/helper/resource/testing_config.go
@@ -214,6 +214,7 @@ func legacyDiffComparisonString(changes *plans.Changes) string {
 	}
 	byModule := map[string]map[string]*ResourceChanges{}
 	resourceKeys := map[string][]string{}
+	requiresReplace := map[string][]string{}
 	var moduleKeys []string
 	for _, rc := range changes.Resources {
 		if rc.Action == plans.NoOp {
@@ -239,6 +240,12 @@ func legacyDiffComparisonString(changes *plans.Changes) string {
 		} else {
 			byModule[moduleKey][resourceKey].Deposed[rc.DeposedKey] = rc
 		}
+
+		rr := []string{}
+		for _, p := range rc.RequiredReplace.List() {
+			rr = append(rr, hcl2shim.FlatmapKeyFromPath(p))
+		}
+		requiresReplace[resourceKey] = rr
 	}
 	sort.Strings(moduleKeys)
 	for _, ks := range resourceKeys {
@@ -254,6 +261,8 @@ func legacyDiffComparisonString(changes *plans.Changes) string {
 		for _, resourceKey := range resourceKeys[moduleKey] {
 			rc := rcs[resourceKey]
 
+			forceNewAttrs := requiresReplace[resourceKey]
+
 			crud := "UPDATE"
 			if rc.Current != nil {
 				switch rc.Current.Action {
@@ -341,7 +350,17 @@ func legacyDiffComparisonString(changes *plans.Changes) string {
 				// at the core layer.
 
 				updateMsg := ""
-				// TODO: Mark " (forces new resource)" in updateMsg when appropriate.
+
+				// This may not be as precise as in the old diff, as it matches
+				// everything under the attribute that was originally marked as
+				// ForceNew, but should help make it easier to determine what
+				// caused replacement here.
+				for _, k := range forceNewAttrs {
+					if strings.HasPrefix(attrK, k) {
+						updateMsg = " (forces new resource)"
+						break
+					}
+				}
 
 				fmt.Fprintf(
 					&mBuf, "  %s:%s %#v => %#v%s\n",