diff --git a/src/pvxs/data.h b/src/pvxs/data.h
index 90b1a41..da4eba9 100644
--- a/src/pvxs/data.h
+++ b/src/pvxs/data.h
@@ -421,6 +421,17 @@ public:
     //! Use this definition as a member (eg. sub-structure) in another definition.
     Member as(const std::string& name) const;
 
+    /** Use this definition as a member (eg. sub-structure) in another definition
+     *  with a (limited) type change.
+     *
+     *  A ``Kind::Compound`` type (eg. ``Struct``) may be changed to another
+     *  Compound type (eg. ``StructA``).  However. changes between Compound
+     *  and non-Compound are not allowed.
+     *
+     *  @since UNRELEASED
+     */
+    Member as(TypeCode code, const std::string& name) const;
+
 private:
     std::shared_ptr<Member> _append_start();
     static
diff --git a/src/type.cpp b/src/type.cpp
index 46d2282..783228f 100644
--- a/src/type.cpp
+++ b/src/type.cpp
@@ -336,6 +336,17 @@ Member TypeDef::as(const std::string& name) const
     return ret;
 }
 
+Member TypeDef::as(TypeCode code, const std::string& name) const
+{
+    Member ret(as(name));
+
+    if((code.kind()==Kind::Compound) ^ (ret.code.kind()==Kind::Compound))
+        throw std::logic_error("as() may change between Compound and non-Compound");
+
+    ret.code = code;
+    return ret;
+}
+
 std::shared_ptr<Member> TypeDef::_append_start()
 {
     if(!top || (top->code.scalarOf()!=TypeCode::Struct && top->code.scalarOf()!=TypeCode::Union))
diff --git a/test/testtype.cpp b/test/testtype.cpp
index 5a90891..77491b7 100644
--- a/test/testtype.cpp
+++ b/test/testtype.cpp
@@ -385,9 +385,10 @@ void testTypeDefAppend()
 {
     testDiag("%s()", __func__);
 
-    auto base = TypeDef(TypeCode::Struct, "epics:nt/NTDummy:0.0", {
-                            members::UInt32("A"),
-                        }).create();
+    TypeDef start(TypeCode::Struct, "epics:nt/NTDummy:0.0", {
+            members::UInt32("A"),
+    });
+    auto base = start.create();
 
     auto sub = TypeDef(base);
     sub += {
@@ -396,13 +397,47 @@ void testTypeDefAppend()
 
     auto amend = sub.create();
 
+    start += {
+            members::UInt32("C"),
+    };
+    auto extend = start.create();
+
     testEq(base.id(), "epics:nt/NTDummy:0.0");
     testEq(base["A"].type(), TypeCode::UInt32);
     testEq(base["B"].type(), TypeCode::Null);
+    testEq(base["C"].type(), TypeCode::Null);
 
     testEq(amend.id(), "epics:nt/NTDummy:0.0");
     testEq(amend["A"].type(), TypeCode::UInt32);
     testEq(amend["B"].type(), TypeCode::UInt32);
+    testEq(amend["C"].type(), TypeCode::Null);
+
+    testEq(extend.id(), "epics:nt/NTDummy:0.0");
+    testEq(extend["A"].type(), TypeCode::UInt32);
+    testEq(extend["B"].type(), TypeCode::Null);
+    testEq(extend["C"].type(), TypeCode::UInt32);
+}
+
+void testTypeDefAppendIncremental()
+{
+    testDiag("%s()", __func__);
+
+    TypeDef part(TypeCode::Struct, {
+                     members::UInt32("x"),
+                 });
+
+    TypeDef whole(TypeCode::Struct, {
+                      members::UInt32("a"),
+                      part.as(TypeCode::StructA, "b"),
+                  });
+
+    testStrEq(std::string(SB()<<whole),
+              "struct {\n"
+              "    uint32_t a\n"
+              "    struct[] {\n"
+              "        uint32_t x\n"
+              "    } b\n"
+              "}\n");
 }
 
 //! Returns the frankenstruct
@@ -554,7 +589,7 @@ void testFormat()
 
 MAIN(testtype)
 {
-    testPlan(56);
+    testPlan(63);
     testSetup();
     showSize();
     testCode();
@@ -562,6 +597,7 @@ MAIN(testtype)
     testTypeDef();
     testTypeDefDynamic();
     testTypeDefAppend();
+    testTypeDefAppendIncremental();
     testOp();
     testFormat();
     cleanup_for_valgrind();