They're missing a bunch of possibilities. Maybe a second is lengthened? Maybe we decouple timekeeping from the rotation of the Earth? Maybe we spin the Earth back up?
Lengthening the second is unlikely: for every physical phenomenon with a duration (from the rotational period of other planets to the frequency of atomic transitions), you would forever after have to wonder, is that value quoted in"new seconds" or "old seconds"?
Redefining units leads to ultimate backwards compatibility problems.