  In this paper we present a new formulation of the change of gauge formulas in
second order cosmological perturbation theory which unifies and simplifies
known results. Our approach is based on defining new second order scalar
perturbation variables by adding a multiple of the square of the corresponding
first order variables to each second order variable. A bonus is that these new
perturbation variables are of broader significance in that they also simplify
the analysis of second order scalar perturbations in the super-horizon regime
in a number of ways, and lead to new conserved quantities.
