Yes, many. I'm going to talk about the British/Dominion armies here as that's what I know. Soldiers who served at the start of the war in the first contingent tended to be promoted out of front line fighting roles and into other positions when the BEF expanded through 1915-16. There were many staff positions to fill in battalions, brigades, divisions and corps and soldiers who had pre-war experience were fast tracked into these positions. They were also taken out of the line and put into training the new armies being formed at home or in training depots in France/Egypt before being rotated back into front line units later on.

There were also small numbers of regular soldiers who survived the entirety of the war in fighting formations, though most were wounded at least once and many several times. The Imperial War Museum has quite a few recorded interviews with veterans of the war, including ones from 1914 who survived It wasn't uncommon for a soldier to serve the entirety of the war, but it very much depended where they served, in what role, what promotions they received and the like. In the AIF the casualty rate of enlisted soldiers was over 60%, but not all of these would be killed or even rendered unfit for service, so many of them would end up going back to their units. The likelihood is that there would have been significant numbers of soldiers on all sides who fought from 1914, but probably far fewer who actually fought in front line units the entire time.