  We propose a new technique to study fast transitions during inflation, by
studying the dynamics of quantum quenches in an $O(N)$ scalar field theory in
de Sitter spacetime. We compute the time evolution of the system using a
non-perturbative large-$N$ limit approach. We derive the self-consistent mass
equation for several physically relevant transitions of the parameters of the
theory, in a slow motion approximation. Our computations reveal that the
effective mass after the quench evolves in the direction of recovering its
value before the quench, but stopping at a different asymptotic value, in which
the mass squared is strictly positive. Furthermore, we tentatively find
situations in which the effective mass squared can be temporarily negative,
thus breaking the $O(N)$ symmetry of the system for a certain time, only to
then come back to a positive value, restoring the symmetry. We argue the
relevance of our new method in a cosmological scenario.
