permanent change doesn't equal impact on the player necessary.
so a temporarily change can be as, if not more of an impact than a permanent change, as long as the temporarily change last long enough to be memorable for the player.
i expect an impact of an experience to be weighted on how compelling and engaging the story is being told.
i also think that a change in a gameworld is only compelling when the player actually feels in the driver seat on such a change, but how can you do that when the change isn't only factored by one but by many more players?
well one way to do that is to give players the opportunity to experience both outcomes of such storyline-change.
as a player never being able to experience another outcome of two potential stoylines feels rather manipulative. a recent example in GW2 history on this is the election of Evon Gnashblade and Ellen Kiel.
we never get to witness Evon Gnasblade way of handling things, even though a dev' stated that even if Evon won it wouldn't had big of an impact...and that reveal just reeks of lazy-work that is uninspiring...
if such a change would be anything meaningful the dev's would have worked out two radical outcomes that would made a huge impact in either one of the chosen paths.
such a potential dynamic in gameworld-change could have been played out, showing the players both of the story-line changes, on the various servers we used to have before the introduction of 'megaservers'. but alas, megaservers would eliminate such potential in branching of storylines across different version of the gameworld-servers.