Yes exactly in NZ it is quite easy.....
91.405 IFR alternate aerodrome requirement
(a) A pilot-in-command of an aircraft operating under IFR must list at
least one alternate aerodrome in the flight plan unless—
(1) the aerodrome of intended landing has a standard instrument
approach procedure published in the applicable AIP; and
(2) at the time of submitting the flight plan, the meteorological
forecasts indicate, for at least 1 hour before and 1 hour after the
estimated time of arrival at the aerodrome of intended landing,
that—
(i) the ceiling at the aerodrome will be at least 1000 feet
above the minima published in the applicable AIP for the
instrument procedure likely to be used; and
(ii) visibility will be at least 5 km, or 2 km more than the
minima published in the applicable AIP, whichever is the
greater.
So I go from AKL to WLG and half way there a new report for 5k and BKN 900' comes out and a corresponding new amended TAF to reflect this I do not have to declare a PAN PAN and continue and file some paperwork. Because at the time of filing I was meeting my regulatory requirements. Now if I did not have much extra gas on board I would be looking at some places along the way but I would not divert there unless there were other aircraft going missed at WLG.
Take each situation as a separate one and you will come up with a different set of results every time