Well, I'd say that Doom Patrol, Metamorpho, and Metal Men were the books that first started looking like Marvel books, DP in particular.

For the rest of the DCU, I'd say the Schwartz titles began to take that shift first in the late 60s. Aquaman did post-Haney, Wonder Woman with the "mod" period, Superman after Weisinger, and Teen Titans around the late 10s, early 20s, a period I don't have covered in my collection. Other titles went gradually as the rest of the DCU did.

In general, the biggest changes were probably happening between 68-70.

-Eric