diff --git a/proto/proto.go b/proto/proto.go
index 325bbc6..5c905d4 100644
--- a/proto/proto.go
+++ b/proto/proto.go
@@ -66,22 +66,57 @@ func (t *AccessKey) ValidateService(service *Service) bool {
 // MarshalJSON adds freeCU, softQuota and hardQuota fields to the json.
 // This keeps compatibility with older versions of QuotaControl.
 // TODO: remove this once all services have migrated to newer version.
-func (l *Limit) MarshalJSON() ([]byte, error) {
+func (l Limit) MarshalJSON() ([]byte, error) {
+	// Alias is used to avoid infinite recursion when marshaling Limit.
 	type Alias Limit
+
 	var v = struct {
-		*Alias
-		CreditsIncluded int64 `json:"freeCU"`
-		SoftQuota       int64 `json:"softQuota"`
-		HardQuota       int64 `json:"hardQuota"`
+		Alias
+		FreeCU    int64 `json:"freeCU"`
+		SoftQuota int64 `json:"softQuota"`
+		HardQuota int64 `json:"hardQuota"`
 	}{
-		Alias:           (*Alias)(l),
-		CreditsIncluded: l.FreeWarn,
-		SoftQuota:       l.OverWarn,
-		HardQuota:       l.OverMax,
+		Alias:     (Alias)(l),
+		FreeCU:    l.FreeWarn,
+		SoftQuota: l.OverWarn,
+		HardQuota: l.OverMax,
 	}
 	return json.Marshal(v)
 }
 
+// UnmarshalJSON parses the json and checks if the older version fields are set
+// and the newer ones are empty. If so, it uses the older fields to populate the new ones.
+func (l *Limit) UnmarshalJSON(b []byte) error {
+	// Alias is used to avoid infinite recursion when marshaling Limit.
+	type Alias Limit
+
+	v := struct {
+		Alias
+		FreeCU    int64 `json:"freeCU"`
+		SoftQuota int64 `json:"softQuota"`
+		HardQuota int64 `json:"hardQuota"`
+	}{}
+	if err := json.Unmarshal(b, &v); err != nil {
+		return err
+	}
+	*l = Limit(v.Alias)
+	if v.FreeCU != 0 {
+		if l.FreeWarn == 0 {
+			l.FreeWarn = v.FreeCU
+		}
+		if l.FreeMax == 0 {
+			l.FreeMax = v.FreeCU
+		}
+	}
+	if v.SoftQuota != 0 && l.OverWarn == 0 {
+		l.OverWarn = v.SoftQuota
+	}
+	if v.HardQuota != 0 && l.OverMax == 0 {
+		l.OverMax = v.HardQuota
+	}
+	return nil
+}
+
 func (l Limit) Validate() error {
 	if l.RateLimit < 1 {
 		return fmt.Errorf("rateLimit must be > 0")
diff --git a/proto/proto_test.go b/proto/proto_test.go
index 0d4d803..56536e8 100644
--- a/proto/proto_test.go
+++ b/proto/proto_test.go
@@ -240,4 +240,12 @@ func TestLimitJSON(t *testing.T) {
 	err = json.Unmarshal(rawJson, &legacyLimit)
 	require.NoError(t, err)
 	assert.Equal(t, legacyExpected, legacyLimit)
+
+	legacyJson, err := json.Marshal(&legacyExpected)
+	require.NoError(t, err)
+
+	limit = proto.Limit{}
+	err = json.Unmarshal(legacyJson, &limit)
+	require.NoError(t, err)
+	assert.Equal(t, expected, limit)
 }
diff --git a/quotacontrol_test.go b/quotacontrol_test.go
index 302124d..edcb6f3 100644
--- a/quotacontrol_test.go
+++ b/quotacontrol_test.go
@@ -298,7 +298,7 @@ func TestDefaultKey(t *testing.T) {
 
 	limit := proto.Limit{
 		RateLimit: 100,
-		FreeWarn:  5,
+		FreeMax:   5,
 		OverWarn:  7,
 		OverMax:   10,
 	}