Frame shift: Terraforming Terra Firma:
The Earth was once a snowball planet, but it's not today. Given the premise that Earth is shifted out of it's orbit into a new, much colder orbit, and humanity can survive the shift (not so cold as to destroy the atmosphere, kill everyone, etc.) why not look at "shift back to it's original orbit" as "shift back to it's original climate?
Given humans and technology, plus a million years to do it in, then there really isn't a better candidate for terraforming than Earth. If people are willing to terraform a relatively miserable ball like Mars, then Earth, with all it's known qualities and resources, would be so much easier to take a Mars-like Earth and convert it into an Earth-like Earth.
All the things people talk about for terraforming Mars work better here. Okay, it might be easier gravitationally to launch ships from the surface of Mars, and most Mars expeditions assume access to shipped-in Earth equipment. But we're assuming a sort of subterranean Earth existence where people have underground cities, factories, hydroponics, etc. Depending on how cold it gets and how much sun you want, plus if it's an elliptical (sometimes warm) orbit, you may have rare summers, possibly even living oceans (under deep ice).
So clever humans make the ground dark, use geothermal heating, and start putting up solar reflectors. We dump greenhouse gasses into the air. We do all the things we were going to do to Mars.
I'm not sure if the environment will fully recover. But I bet we could turn an Earth-sized Mars with all our stuff already here into a fairly respectable place to live. Given enough time.