Multi-parameter type classes are more flexible. Here is how you can write your old code:
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
class (ClassA a, ClassB b) => ClassC a b where from :: a -> [b] to :: a -> [b]
data H = H
class ClassA a where toInt :: a -> Int class ClassB b where fromInt :: Int -> b
instance ClassB H where fromInt _ = H
data Test = Test { m :: H } instance ClassA Test where toInt _ = 0
instance ClassC Test H where from = (:[]) . m to = (:[]) . m
The constraints in the ClassC a b declaration specify that in all instances of ClassC, the type a must be in ClassA and the type b must be in ClassB. This is the case for the "ClassC Test H" instance. You can also specify that for some particular 'a' the function 'from' can produce the value of the type [b] for any b in ClassB. The caller will determine which b it wants. This is similar to your original intention, as I understand.
instance ClassA Int where toInt = id
instance ClassB b => ClassC Int b where from x = [fromInt x]
t1:: [H] t1 = from (5::Int)