  We analyze quantum quenches in integrable models and in particular the
determination of the initial state in the basis of eigenstates of the
post-quench hamiltonian. This leads us to consider the set of transformations
of creation and annihilation operators that respect the Zamolodchikov-Faddeev
algebra satisfied by integrable models. We establish that the Bogoliubov
transformations hold only in the case of quantum quenches in free theories. In
the most general case of interacting theories, we identify two classes of
transformations. The first class induces a change in the S-matrix of the theory
but not of its ground state, whereas the second class results in a "dressing"
of the operators. As examples of our approach we consider the transformations
associated with a change of the interaction in the Sinh-Gordon and the
Lieb-Liniger model.
