Since this is entirely hypothetical, nobody can tell you.
Whenever anybody discusses immigration (to any country), I think there is always an element of fear among those who are doing it, that their situation is not assured, in some way or other, and the rumours that go round about PR one day not being permanent are part of that insecurity. Also, friends and family that would rather the migrants didn't go often tend to feed the rumours. But they are NOT FACT.
The NZ government has not made any statement that they intend to make a change in the law. If they DID make a change in the law, that change would and could be, presumably, whatever they wanted it to be at the time, decided by the then MPs and what they voted through. Considering MPs are elected representatives, they would have to bear in mind the opinions of their constituents, including the immigrants among them. Any discussion of what could happen is bound to be full of 'if', 'they might', and 'maybe'. For now, regardless of the scare-mongering rumours, PR is permanent.