There are several major shifts in the poem. In lines 1-8, Owen describes the horrible battle fatigue the men must endure in during World War I. They are described as " old beggars under sacks," and "drunk with fatigue". Many are marching "asleep". Suddenly, the tone changes at the beginning of stanza 2, in line 9 when someone yells "Gas! GAS!" Now every soldier awake with fear as they engage in "an ecstasy of fumbling" to put on their gas masks. Every man succeeds except for one soldier who the speaker sees "drowning" under a green sea of poison gas. In line 14, the tone then changes to a somber, horror stricken description of the man dying. He is "guttering, choking, drowning". His face writhes in pain and blood comes "gargling from the froth-corrupted lungs." Finally the tone changes for the final time in line 25 as the speaker reflects on what he has just seen. Using an allusion to an old Roman battle cry, the speaker rejects the enthusiastic patriotism that lead many soldiers to enlist in the war. He says calls the battle cry of 'dulce et decorum est pro patria mori" a lie and observes that once you have seen someone die of a gas attack you would never again say that "it is sweet and honorable to die for one's country."
The "shift" that you speak of in "Dulce Et Decorum Est" is in line nine when the speaker exclaims, "Gas! Gas! Quick, boys!--" Before this point the poem is simply a description (although a morose one) of marching in formation to some undisclosed location during World War I. However, as soon as Owen's exclamation rings in, the poem totally changes. The gas, so commonly used during the war, damages no one who got his mask on in time, but others weren't so lucky. Now the description is no longer of marching, but of death and dying. "Watch the white eyes writhing in his face, / His hanging face, like a devil's sick of sin; / If you could hear, at every jolt, the blood / Come gargling from the froth-corrupted lungs, / Obscene as cancer," and, thus, Wilfred Owen makes his readers highly aware of the atrocities of war.