[Haskell-cafe] Copying a syntax tree

Hi! I have a syntax tree defined like this:

data A a = A Int Int (B a)

data B a = B String String (C a)

data C a = C Int Int (D a)

and so on, all the data are parametrized with a type variable. This variable is actually used as a field in the very end of a hierarchy:

data D a = D Int a

Now I have to write a function which would copy (A Int) to (A String). Is it possible to do so using TH/syb without writing copyA (A i1 i2 b) = A i1 i2 (copyB b) copyB = ... copyC = ... .. copyD (D i a) = D i (show a) ? Could you provide me with a hint? Thanks, Sergey