diff --git a/src/plugins/projectexplorer/kitinformation.cpp b/src/plugins/projectexplorer/kitinformation.cpp
index 1e0f8f73055..d84a7b69abd 100644
--- a/src/plugins/projectexplorer/kitinformation.cpp
+++ b/src/plugins/projectexplorer/kitinformation.cpp
@@ -58,6 +58,8 @@
 
 namespace ProjectExplorer {
 
+const char KITINFORMATION_ID_V1[] = "PE.Profile.ToolChain";
+const char KITINFORMATION_ID_V2[] = "PE.Profile.ToolChains";
 const char KITINFORMATION_ID_V3[] = "PE.Profile.ToolChainsV3";
 
 // --------------------------------------------------------------------------
@@ -387,6 +389,65 @@ Tasks ToolChainKitAspect::validate(const Kit *k) const
 void ToolChainKitAspect::upgrade(Kit *k)
 {
     QTC_ASSERT(k, return);
+
+    const Utils::Id oldIdV1 = KITINFORMATION_ID_V1;
+    const Utils::Id oldIdV2 = KITINFORMATION_ID_V2;
+
+    // upgrade <=4.1 to 4.2 (keep old settings around for now)
+    {
+        const QVariant oldValue = k->value(oldIdV1);
+        const QVariant value = k->value(oldIdV2);
+        if (value.isNull() && !oldValue.isNull()) {
+            QVariantMap newValue;
+            if (oldValue.type() == QVariant::Map) {
+                // Used between 4.1 and 4.2:
+                newValue = oldValue.toMap();
+            } else {
+                // Used up to 4.1:
+                newValue.insert(Deprecated::Toolchain::languageId(Deprecated::Toolchain::Cxx), oldValue.toString());
+
+                const Utils::Id typeId = DeviceTypeKitAspect::deviceTypeId(k);
+                if (typeId == Constants::DESKTOP_DEVICE_TYPE) {
+                    // insert default C compiler which did not exist before
+                    newValue.insert(Deprecated::Toolchain::languageId(Deprecated::Toolchain::C),
+                                    defaultToolChainIds().value(Utils::Id(Constants::C_LANGUAGE_ID)));
+                }
+            }
+            k->setValue(oldIdV2, newValue);
+            k->setSticky(oldIdV2, k->isSticky(oldIdV1));
+        }
+    }
+
+    // upgrade 4.2 to 4.3 (keep old settings around for now)
+    {
+        const QVariant oldValue = k->value(oldIdV2);
+        const QVariant value = k->value(ToolChainKitAspect::id());
+        if (value.isNull() && !oldValue.isNull()) {
+            QVariantMap newValue = oldValue.toMap();
+            QVariantMap::iterator it = newValue.find(Deprecated::Toolchain::languageId(Deprecated::Toolchain::C));
+            if (it != newValue.end())
+                newValue.insert(Utils::Id(Constants::C_LANGUAGE_ID).toString(), it.value());
+            it = newValue.find(Deprecated::Toolchain::languageId(Deprecated::Toolchain::Cxx));
+            if (it != newValue.end())
+                newValue.insert(Utils::Id(Constants::CXX_LANGUAGE_ID).toString(), it.value());
+            k->setValue(ToolChainKitAspect::id(), newValue);
+            k->setSticky(ToolChainKitAspect::id(), k->isSticky(oldIdV2));
+        }
+    }
+
+    // upgrade 4.3-temporary-master-state to 4.3:
+    {
+        const QVariantMap valueMap = k->value(ToolChainKitAspect::id()).toMap();
+        QVariantMap result;
+        for (const QString &key : valueMap.keys()) {
+            const int pos = key.lastIndexOf('.');
+            if (pos >= 0)
+                result.insert(key.mid(pos + 1), valueMap.value(key));
+            else
+                result.insert(key, valueMap.value(key));
+        }
+        k->setValue(ToolChainKitAspect::id(), result);
+    }
 }
 
 void ToolChainKitAspect::fix(Kit *k)
diff --git a/src/plugins/projectexplorer/toolchain.cpp b/src/plugins/projectexplorer/toolchain.cpp
index 1739fc5e890..69355e44130 100644
--- a/src/plugins/projectexplorer/toolchain.cpp
+++ b/src/plugins/projectexplorer/toolchain.cpp
@@ -44,6 +44,7 @@ using namespace Utils;
 static const char ID_KEY[] = "ProjectExplorer.ToolChain.Id";
 static const char DISPLAY_NAME_KEY[] = "ProjectExplorer.ToolChain.DisplayName";
 static const char AUTODETECT_KEY[] = "ProjectExplorer.ToolChain.Autodetect";
+static const char LANGUAGE_KEY_V1[] = "ProjectExplorer.ToolChain.Language"; // For QtCreator <= 4.2
 static const char LANGUAGE_KEY_V2[] = "ProjectExplorer.ToolChain.LanguageV2"; // For QtCreator > 4.2
 
 namespace ProjectExplorer {
@@ -86,8 +87,41 @@ public:
     ToolChain::HeaderPathsCache m_headerPathsCache;
 };
 
-} // Internal
 
+// Deprecated used from QtCreator <= 4.2
+
+Utils::Id fromLanguageV1(int language)
+{
+    switch (language)
+    {
+    case Deprecated::Toolchain::C :
+        return Utils::Id(Constants::C_LANGUAGE_ID);
+    case Deprecated::Toolchain::Cxx:
+        return Utils::Id(Constants::CXX_LANGUAGE_ID);
+    case Deprecated::Toolchain::None:
+    default:
+        return Utils::Id();
+    }
+}
+
+} // namespace Internal
+
+namespace Deprecated {
+namespace Toolchain {
+QString languageId(Language l)
+{
+    switch (l) {
+    case Language::None:
+        return QStringLiteral("None");
+    case Language::C:
+        return QStringLiteral("C");
+    case Language::Cxx:
+        return QStringLiteral("Cxx");
+    };
+    return QString();
+}
+} // namespace Toolchain
+} // namespace Deprecated
 
 /*!
     \class ProjectExplorer::ToolChain
@@ -219,6 +253,15 @@ QVariantMap ToolChain::toMap() const
     result.insert(QLatin1String(ID_KEY), idToSave);
     result.insert(QLatin1String(DISPLAY_NAME_KEY), displayName());
     result.insert(QLatin1String(AUTODETECT_KEY), isAutoDetected());
+    // <Compatibility with QtC 4.2>
+    int oldLanguageId = -1;
+    if (language() == ProjectExplorer::Constants::C_LANGUAGE_ID)
+        oldLanguageId = 1;
+    else if (language() == ProjectExplorer::Constants::CXX_LANGUAGE_ID)
+        oldLanguageId = 2;
+    if (oldLanguageId >= 0)
+        result.insert(LANGUAGE_KEY_V1, oldLanguageId);
+    // </Compatibility>
     result.insert(QLatin1String(LANGUAGE_KEY_V2), language().toSetting());
     if (!d->m_targetAbiKey.isEmpty())
         result.insert(d->m_targetAbiKey, d->m_targetAbi.toString());
@@ -328,6 +371,8 @@ bool ToolChain::fromMap(const QVariantMap &data)
             d->m_language = Utils::Id::fromString(langId.mid(pos + 1));
         else
             d->m_language = Utils::Id::fromString(langId);
+    } else if (data.contains(LANGUAGE_KEY_V1)) { // Import from old settings
+        d->m_language = Internal::fromLanguageV1(data.value(QLatin1String(LANGUAGE_KEY_V1)).toInt());
     }
 
     if (!d->m_language.isValid())
diff --git a/src/plugins/projectexplorer/toolchain.h b/src/plugins/projectexplorer/toolchain.h
index a2d5494f87b..6c85293433b 100644
--- a/src/plugins/projectexplorer/toolchain.h
+++ b/src/plugins/projectexplorer/toolchain.h
@@ -51,6 +51,18 @@ namespace ProjectExplorer {
 
 namespace Internal { class ToolChainPrivate; }
 
+namespace Deprecated {
+// Deprecated in 4.3:
+namespace Toolchain {
+enum Language {
+    None = 0,
+    C,
+    Cxx
+};
+QString languageId(Language l);
+} // namespace Toolchain
+} // namespace Deprecated
+
 class ToolChainConfigWidget;
 class ToolChainFactory;
 class Kit;