diff --git a/README.md b/README.md
index c30173d..bc65fb1 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,26 @@ expr, err := spdx.ParseStrict("MIT OR Apache-2.0")  // succeeds
 expr, err := spdx.ParseStrict("Apache 2 OR MIT")    // fails
 ```
 
+### Rewrite expression identifiers
+
+Parse an expression once, then replace its identifiers while keeping its
+operators and precedence:
+
+```go
+expr, err := spdx.ParseStrict(
+    "MIT OR GPL-2.0-only WITH Classpath-exception-2.0",
+)
+rewritten := spdx.RewriteIdentifiers(expr, func(id string) string {
+    return strings.ToLower(id)
+})
+fmt.Println(rewritten)
+// "mit OR (gpl-2.0-only WITH classpath-exception-2.0)"
+```
+
+The callback receives license identifiers, exception identifiers, and complete
+`LicenseRef` or `DocumentRef` values in expression order. Replacement values
+are not validated as SPDX identifiers.
+
 ### Validate licenses
 
 ```go
diff --git a/parse.go b/parse.go
index 6aeca34..c1b2301 100644
--- a/parse.go
+++ b/parse.go
@@ -13,13 +13,14 @@ type Expression interface {
 	String() string
 	// Licenses returns all license identifiers in the expression.
 	Licenses() []string
+	rewriteIdentifiers(func(string) string) Expression
 	isExpr()
 }
 
 // License represents a single SPDX license identifier.
 type License struct {
-	ID       string // The canonical license ID
-	Plus     bool   // True if followed by +
+	ID        string // The canonical license ID
+	Plus      bool   // True if followed by +
 	Exception string // Exception ID if using WITH
 }
 
diff --git a/rewrite.go b/rewrite.go
new file mode 100644
index 0000000..f919245
--- /dev/null
+++ b/rewrite.go
@@ -0,0 +1,47 @@
+package spdx
+
+// RewriteIdentifiers applies rewrite to each license identifier, exception
+// identifier, and complete license reference in expression order. Operators,
+// modifiers, and operator precedence are preserved. Replacements are inserted
+// verbatim and are not validated as SPDX identifiers.
+//
+// A nil rewrite function returns expression.String().
+func RewriteIdentifiers(expression Expression, rewrite func(string) string) string {
+	if rewrite == nil {
+		return expression.String()
+	}
+	return expression.rewriteIdentifiers(rewrite).String()
+}
+
+func (license *License) rewriteIdentifiers(rewrite func(string) string) Expression {
+	rewritten := &License{
+		ID:   rewrite(license.ID),
+		Plus: license.Plus,
+	}
+	if license.Exception != "" {
+		rewritten.Exception = rewrite(license.Exception)
+	}
+	return rewritten
+}
+
+func (reference *LicenseRef) rewriteIdentifiers(rewrite func(string) string) Expression {
+	return &License{ID: rewrite(reference.String())}
+}
+
+func (expression *AndExpression) rewriteIdentifiers(rewrite func(string) string) Expression {
+	return &AndExpression{
+		Left:  expression.Left.rewriteIdentifiers(rewrite),
+		Right: expression.Right.rewriteIdentifiers(rewrite),
+	}
+}
+
+func (expression *OrExpression) rewriteIdentifiers(rewrite func(string) string) Expression {
+	return &OrExpression{
+		Left:  expression.Left.rewriteIdentifiers(rewrite),
+		Right: expression.Right.rewriteIdentifiers(rewrite),
+	}
+}
+
+func (special *SpecialValue) rewriteIdentifiers(func(string) string) Expression {
+	return &SpecialValue{Value: special.Value}
+}
diff --git a/rewrite_external_test.go b/rewrite_external_test.go
new file mode 100644
index 0000000..0c1f29d
--- /dev/null
+++ b/rewrite_external_test.go
@@ -0,0 +1,25 @@
+package spdx_test
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/git-pkgs/spdx"
+)
+
+type expressionWrapper struct {
+	spdx.Expression
+}
+
+func TestRewriteIdentifiersEmbeddedExpression(t *testing.T) {
+	t.Parallel()
+
+	expression, err := spdx.ParseStrict("MIT OR Apache-2.0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	wrapped := expressionWrapper{Expression: expression}
+	if got := spdx.RewriteIdentifiers(wrapped, strings.ToLower); got != "mit OR apache-2.0" {
+		t.Errorf("RewriteIdentifiers = %q", got)
+	}
+}
diff --git a/rewrite_test.go b/rewrite_test.go
new file mode 100644
index 0000000..0decfde
--- /dev/null
+++ b/rewrite_test.go
@@ -0,0 +1,151 @@
+package spdx
+
+import (
+	"slices"
+	"strings"
+	"testing"
+)
+
+func TestRewriteIdentifiers(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name        string
+		input       string
+		rewrite     map[string]string
+		want        string
+		identifiers []string
+	}{
+		{
+			name:    "license",
+			input:   "BSD-3-Clause",
+			rewrite: map[string]string{"BSD-3-Clause": "bsd-new"},
+			want:    "bsd-new",
+			identifiers: []string{
+				"BSD-3-Clause",
+			},
+		},
+		{
+			name:  "compound expression",
+			input: "MIT OR Apache-2.0 AND BSD-3-Clause",
+			rewrite: map[string]string{
+				"MIT":          "mit",
+				"Apache-2.0":   "apache-2.0",
+				"BSD-3-Clause": "bsd-new",
+			},
+			want: "mit OR (apache-2.0 AND bsd-new)",
+			identifiers: []string{
+				"MIT",
+				"Apache-2.0",
+				"BSD-3-Clause",
+			},
+		},
+		{
+			name:  "plus and exception",
+			input: "GPL-2.0+ OR GPL-2.0-only WITH Classpath-exception-2.0",
+			rewrite: map[string]string{
+				"GPL-2.0":                 "gpl-2.0",
+				"GPL-2.0-only":            "gpl-2.0",
+				"Classpath-exception-2.0": "classpath-exception-2.0",
+			},
+			want: "gpl-2.0+ OR (gpl-2.0 WITH classpath-exception-2.0)",
+			identifiers: []string{
+				"GPL-2.0",
+				"GPL-2.0-only",
+				"Classpath-exception-2.0",
+			},
+		},
+		{
+			name:  "license references",
+			input: "LicenseRef-scancode-mit AND DocumentRef-vendor:LicenseRef-custom",
+			rewrite: map[string]string{
+				"LicenseRef-scancode-mit":              "mit",
+				"DocumentRef-vendor:LicenseRef-custom": "unknown-spdx",
+			},
+			want: "mit AND unknown-spdx",
+			identifiers: []string{
+				"LicenseRef-scancode-mit",
+				"DocumentRef-vendor:LicenseRef-custom",
+			},
+		},
+		{
+			name:  "repeated identifier",
+			input: "MIT AND MIT",
+			rewrite: map[string]string{
+				"MIT": "mit",
+			},
+			want:        "mit AND mit",
+			identifiers: []string{"MIT", "MIT"},
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+
+			expression, err := ParseStrict(test.input)
+			if err != nil {
+				t.Fatal(err)
+			}
+			original := expression.String()
+			var identifiers []string
+			got := RewriteIdentifiers(expression, func(identifier string) string {
+				identifiers = append(identifiers, identifier)
+				return test.rewrite[identifier]
+			})
+			if got != test.want {
+				t.Errorf("RewriteIdentifiers(%q) = %q, want %q", test.input, got, test.want)
+			}
+			if !slices.Equal(identifiers, test.identifiers) {
+				t.Errorf("identifiers = %q, want %q", identifiers, test.identifiers)
+			}
+			if expression.String() != original {
+				t.Errorf("RewriteIdentifiers mutated expression: got %q, want %q", expression, original)
+			}
+		})
+	}
+}
+
+func TestRewriteIdentifiersSpecialValues(t *testing.T) {
+	t.Parallel()
+
+	for _, input := range []string{"NONE", "NOASSERTION"} {
+		expression, err := ParseStrict(input)
+		if err != nil {
+			t.Fatal(err)
+		}
+		got := RewriteIdentifiers(expression, func(identifier string) string {
+			t.Fatalf("rewrite called with %q", identifier)
+			return ""
+		})
+		if got != input {
+			t.Errorf("RewriteIdentifiers(%q) = %q", input, got)
+		}
+	}
+}
+
+func TestRewriteIdentifiersNilFunction(t *testing.T) {
+	t.Parallel()
+
+	expression, err := ParseStrict("MIT OR Apache-2.0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got := RewriteIdentifiers(expression, nil); got != expression.String() {
+		t.Errorf("RewriteIdentifiers with nil function = %q", got)
+	}
+}
+
+func BenchmarkRewriteIdentifiers(b *testing.B) {
+	expression, err := ParseStrict(
+		"MIT OR GPL-2.0-only WITH Classpath-exception-2.0",
+	)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.ReportAllocs()
+	for b.Loop() {
+		RewriteIdentifiers(expression, strings.ToLower)
+	}
+}