  Accurate Monte Carlo simulations for high-energy events at CERN's Large
Hadron Collider, are very expensive, both from the computing and storage points
of view. We describe a method that allows to consistently re-use parton-level
samples accurate up to NLO in QCD under different theoretical hypotheses. We
implement it in MadGraph5_aMC@NLO and show its validation by applying it to
several cases of practical interest for the search of new physics at the LHC.
