How are change and transformation presented in "Dulce et Decorum Est" by Wilfred Owen?

This is actually a tough question, because much of the change and transformation in the poem is rather abstract. In "Dulce et Decorum Est," Wilfred Owen chronicles the effects of a WWI gas attack, focusing especially on the mental trauma that this experience has on those who survive the attack but witness the unceremonious death of their comrades. In response, Owen's speaker denounces the traditional idea that dying in battle for one's country is honorable: 

My friend, you would not tell with such high zest 
To children ardent for some desperate glory, 
The old Lie: Dulce et decorum est 


Pro patria mori. (25-28)

The Latin quote, originally from Horace, roughly translates to mean "it is sweet and proper to die for the fatherland." This sentiment is a traditional patriotic ethos, one that presents dying during military service as the ultimate honor. Owen's speaker, however, denounces this idea as a lie.
 
In the trenches of WWI, death was often gruesome and meaningless, and the soldiers on both sides of the conflict (who, we should acknowledge, died in horrifically large numbers due to the rapid advancement of military technology) often felt as if they were fighting and dying for nothing. In this war, dying for the fatherland seemed devoid of meaning or honor. Thus, the change and transformation we see here is one of ideology, as Owen's speaker rejects old ways of thinking about war and forces us to see the gruesome realities behind patriotic idealism. 
 
https://www.poetryfoundation.org/poems/46560/dulce-et-decorum-est