diff --git a/src/scripting/vmthunks.cpp b/src/scripting/vmthunks.cpp
index 4bf34ede8..18d9b214e 100644
--- a/src/scripting/vmthunks.cpp
+++ b/src/scripting/vmthunks.cpp
@@ -263,6 +263,28 @@ DEFINE_ACTION_FUNCTION_NATIVE(FStringStruct, ToLower, StringToLower)
 	return 0;
 }
 
+static void StringMakeUpper(FString *self, FString *out)
+{
+	*out = self->MakeUpper();
+}
+
+DEFINE_ACTION_FUNCTION_NATIVE(FStringStruct, MakeUpper, StringMakeUpper)
+{
+	PARAM_SELF_STRUCT_PROLOGUE(FString);
+	ACTION_RETURN_STRING(self->MakeUpper());
+}
+
+static void StringMakeLower(FString *self, FString *out)
+{
+	*out = self->MakeLower();
+}
+
+DEFINE_ACTION_FUNCTION_NATIVE(FStringStruct, MakeLower, StringMakeLower)
+{
+	PARAM_SELF_STRUCT_PROLOGUE(FString);
+	ACTION_RETURN_STRING(self->MakeLower());
+}
+
 static int StringToInt(FString *self, int base)
 {
 	return (int)self->ToLong(base);
diff --git a/src/utility/zstring.cpp b/src/utility/zstring.cpp
index 6bcf36c7f..efb79a460 100644
--- a/src/utility/zstring.cpp
+++ b/src/utility/zstring.cpp
@@ -40,6 +40,7 @@
 #include "zstring.h"
 #include "v_text.h"
 #include "utf8.h"
+#include "fontinternals.h"
 
 FNullStringData FString::NullString =
 {
@@ -680,22 +681,30 @@ void FString::ToLower ()
 	UnlockBuffer();
 }
 
-void FString::SwapCase ()
+FString FString::MakeLower()
 {
-	LockBuffer();
-	size_t max = Len();
-	for (size_t i = 0; i < max; ++i)
+	TArray<uint8_t> builder(Len());
+	int pos = 0;
+	while (int c = GetNextCharacter(pos))
 	{
-		if (isupper(Chars[i]))
-		{
-			Chars[i] = (char)tolower(Chars[i]);
-		}
-		else
-		{
-			Chars[i] = (char)toupper(Chars[i]);
-		}
+		if (c < 65536) c = lowerforupper[c];
+		auto cp = MakeUTF8(c);
+		while (auto uc = *cp++) builder.Push(uc);
 	}
-	UnlockBuffer();
+	return FString(builder);
+}
+
+FString FString::MakeUpper()
+{
+	TArray<uint8_t> builder(Len());
+	int pos = 0;
+	while (int c = GetNextCharacter(pos))
+	{
+		if (c < 65536) c = upperforlower[c];
+		auto cp = MakeUTF8(c);
+		while (auto uc = *cp++) builder.Push(uc);
+	}
+	return FString(builder);
 }
 
 void FString::StripLeft ()
diff --git a/src/utility/zstring.h b/src/utility/zstring.h
index 35e7be700..9c7a2ffd1 100644
--- a/src/utility/zstring.h
+++ b/src/utility/zstring.h
@@ -238,7 +238,8 @@ public:
 
 	void ToUpper ();
 	void ToLower ();
-	void SwapCase ();
+	FString MakeUpper();
+	FString MakeLower();
 
 	void StripLeft ();
 	void StripLeft (const FString &charset);
diff --git a/wadsrc/static/zscript/base.zs b/wadsrc/static/zscript/base.zs
index 18a80287b..0938c9ab8 100644
--- a/wadsrc/static/zscript/base.zs
+++ b/wadsrc/static/zscript/base.zs
@@ -905,8 +905,10 @@ struct StringStruct native
 	native int IndexOf(String substr, int startIndex = 0) const;
 	deprecated("3.5.1") native int LastIndexOf(String substr, int endIndex = 2147483647) const;
 	native int RightIndexOf(String substr, int endIndex = 2147483647) const;
-	native void ToUpper();
-	native void ToLower();
+	deprecated("4.1") native void ToUpper();
+	deprecated("4.1") native void ToLower();
+	native String MakeUpper();
+	native String MakeLower();
 	native int ToInt(int base = 0) const;
 	native double ToDouble() const;
 	native void Split(out Array<String> tokens, String delimiter, EmptyTokenType keepEmpty = TOK_KEEPEMPTY) const;