One thing I've noticed is that they only seem to migrate to relatively close systems, not from one side of the galaxy to another instantly.
Not sure if other people have noticed this.
Say I have 2 pops on a planet, one that can breathe and one that cannot. If I remove the population that cannot breathe, will the domes (limitation) disappear the next turn?
How does this work with new colonies? If I have a pop in my empire that can breathe the atmosphere, will it appear green? Do I have to colonize, move them (or allow them to migrate), then remove my pop?