CYK result de-transformation

Suppose we have a rules derived from a treebank.
And in order to get a syntax tree of a given sentence we use the cyk algorithm.
In order to use the cyk we should convert the rules into chomsky normal form,
Such that all of my rules are A -> B C or A -> a (A,B,C are non-terminals and a is a terminal).
From the cyk we get the most probable tree, constituted from the cnf rules.
But what if we want to de-transform the tree to be constituted from the original rules?
We can expand very easily the Binary rules into the original.
But what about unit productions? for example; A->B->a transformed into A->a in cnf, how do we get the original unary rules path A->B->a?
Any ideas will be helpful.