I see other men named Dalrymple as enlisted in U.S. Marines from 1875 to 1940's when database years ends. Also in the VA BIRLS death index with birth years 1890 to 1900 and later for all services. So it was not the Military that required the change. When and where did he enlist? He either made a personal choice or had a something in his past that would disqualify him.
Have you found a WWI Draft Registration Card for him? Which name?