  Recent developments have made possible the computation of equilibrium
dynamical correlators in quantum impurity problems. In many situations however,
one is rather interested in correlators subject to a non equilibrium initial
preparation; this is the case for instance for the occupation probability
$P(t)$ in the double well problem of dissipative quantum mechanics (DQM). We
show in this paper how to handle this situation in the framework of integrable
quantum field theories by introducing ``boundary interactions changing
operators''. We determine the properties of these operators by using an
axiomatic approach similar in spirit to what is done for form-factors. This
allows us to obtain new exact results for $P(t)$; for instance, we find that
that at large times (or small $g$), the leading behaviour for $g < 1/2}$ is
$P(t)\propto e^{-\Gamma t}\cos\Omega t$, with the universal ratio.
$\Omega/\Gamma = \cot {\pi g}/{2(1-g)}$.
