The Worm In Waiting doesn't change your government-level Ethos, just that of your POPs.The Old Shrine event chain allows to change the ethics to Fanatical Spiritualist/Collectivist and the Worm is Wainting allows to change to Fanatical Militarist/Individualist or Fanatical Pacifist/Individualist if you were militarist in the begining.
I don't think there are other events that allow to change it.
I tought that as changed the Ethos of you primary species it would allow to change the government(I never actually reached until the very end of the Worm)The Worm In Waiting doesn't change your government-level Ethos, just that of your POPs.
*cough* shrine of the old god does an empire wide changeFor 1.4.1 and any earlier version:
You can not. Aside from becomming the thrall of the Awakened Spiritualist Fallen Empire (Collectivsit, Fanatic Spiritualist) there is now way.
There are a few Events that can change your Pops Ethos. But this does not have any effect on your Empire Ethos, so you end up with very divergent pops.
Maybe console commands could do it? Have to try that one time.
For the comming Update - 1.5 or Banks - the ability has been confirmed. As part of the totally new Ethos System.
Not in 1.4.I tought that as changed the Ethos of you primary species it would allow to change the government(I never actually reached until the very end of the Worm)