Can a neurosurgeon become a plastic surgeon? How long would it take to do that?

A neurosurgeon cannot directly become a plastic surgeon. For a change, one would have to undergo an MS or DNB in General Surgery and then MCh/DNB in Plastic Surgery.
It will be another 6–8 years, which is an unusual career change. It is wiser to be a bit more specialised in one's own branch of surger