The knowledge he has of the "possible" future is NOT arcane knowledge that would alter this timeline. Rather, Old Spock is now a part of this timeline. The timeline has already been changed. Where it goes from here depends on how agents (Old Spock included) decide to act. He would only be changing the timeline in the mundane sense in which we all change the future when we act on the basis of information. Warning Starfleet about V'Ger would not undo an existing reality, because Old Spock's decisions in this timeline have yet to form what that reality will be. Old Spock's superior data about threats is supremely useful information which he could share to save billions of lives (let's bracket the question of time-traveling to save Vulcan).