It's not like human cultures haven't done just about everything that their physical environment and technological ability has allowed to do. And other eartly life forms also colonise every single niche they have physical access to.

Can the last politician to go out the revolving door please turn the lights off?

Lets make an experience of thought:
If the atmosphere were to become inhospitable before we can leave the planet, and the oceans remained habitable, those seeking to survive would go there. At first in pressurized habitats. And when they realize their chances of maintaining their civilizations would be greatly enhanced by bolder moves, they will improve their biological capacity to live underwater. First by resisting higher pressures, then by going cold blooded (to withstand the heat loss in cold water and still be able to recover), then by breathing with gills (although the water would have to be hyper-oxygenated to enable 37°C body temperature and full brain performance)

I don't think changes will be made just for the fun of it. But if you must leave your planet for another one, and maximize the survival of your descendant on new worlds, GM quickly becomes the most effective way to achieve this.

Pierre