[MegaTrain]

Nothing happens. If you move non-breathing population to an existing world, it will simply maintain the existing levels of population, stored items, and facilities.

You can't add any more, of course, and population growth will be 0. It doesn't kill your people or eject your cargo, though (fortunately).

You can actually see this under a few conditions:
1-Move a non-breathing population to a planet.
2-Capture or trade a world from someone who has "Advanced Storage" racial tech.
3-There is a bug where under the right conditions you can get a non-breathable starting homeworld. I've started with a huge planet with 8000 population, max 500.