Easy as pie! (Pumpkin, given the season!
)
All you need to do is send a transport with the right population type, but not 100 percent full, and with enough space to hold the entire current population. If the planet is full, move some of them into the transport and replace with some of the hydrogen breathers. Then swap the remaining oxygen breathers into the transport or as many as you can. Keep swapping until you've got a planet with only hydrogen breathers. Once there are no oxygen breathers left on the planet it will be "full" capacity instead of domed. No special measures necessary. It won't let you remove ALL of the population from a planet anyway.
[This message has been edited by Baron Munchausen (edited 22 November 2000).]