[Baron Munchausen]

Yep, ordinary cargo transfer. Put at least some of your 'native' breathing race on the planet, then you can move all of the CO2 breathers off. When no more 'non-native' breathers are present the available planet space will be full sized instead of domed.

No you cannot 'repatriate' them unless you want to 'gift' a transport to the Phong with them onboard. Using them to make a CO2 atmosphere planet 'undomed' is much more productive anyway.