But from Picards point of view it had, so they were now occupying the timeline where the change had occurred with its ripples into the past. Picard isn't trying to prevent the change, it had already happened/ will happen, he is trying to make a change to the modified timeline to create the original timeline he is familiar with. Because in the original future Picard had traveled back past the point of change, and in the new future this did not happen this would mean both the future and past has been altered, so that although the date of change still lies in the future it has already happened thereby changing the past.