The Big Time

From back cover Ace paperback December 1982:

This is war: The biggest, longest war that anyone could imagine. The soldiers are recruited at the moment of death to fight through all of time. The goal is to change the past, and insure victory in the future.

The Change Winds are blowing. Welcome to the Big Time.

"Have you ever worried about your memory, because it doesn't seem to be bringing you the same picture of the past from one day to the next? Have you ever been afraid that your personality was changing because of forces beyond your knowledge or control? Have you ever felt sure that sudden death was about to jump you from nowhere? Have you ever thought that the whole universe might be a crazy mixed-up dream? If you have, you've had hints of the Change War."

Synopsis from Open Library.