[fragmede]

Judging by our behavior on the current rock we're on, don't give them the choice. Humanity has been around for a long time. In that time, I don't believe I've ever heard discussion of changing the direction the Sun and Earth are headed. If there were something deep inside the Earth, or another planet that would activate in another 10,000 years that would do the phone home step, we still don't know about it. So while we're hypothesizing about sci-fi Von Neumann probes, just scale the entire thing up to the size of a solar system and send that off in the right direction. Collect all the mass in our system that isn't Sol, Terra, (and Luna), and build a rogue planet with an underground civilization, and then don't give them engines. Unless the civilization on the planet advances too the point that they can create engines, decide they don't want to go where you've sent them, voila, generational ship. Just bury a computer deep I side the rogue planet's core that activates once it reaches the destination.