If you choose to make a new character, your old character will remain in the colony. If your new character dies, you will again have the choice between reactivating your old character or creating a new one.
So, if now that you know this you feel like making a new character and leave your old one in the colony, but you've already reactivated your old character, I will give you a one-time offer of resetting that choice. Just post your reactivated character's name here.
As far as I know, we will not be able to switch characters at will, however. Neither during flight nor in the downtimes, only if a character suffers an untimely death. So think on it for a while before making your choice