Once a wavefunction has collapsed, can the system change to a new state if we disturb the system? For example, if we have a particle in a state of well defined energy (e.g. ground state) and then suddenly change the potential of the particle, can it change to a new state (e.g. excited state)? How could one work out the probability of it changing state after the disturbance? Thanks, I'd be grateful if you can contribute to all or just part of my question.