The idea of the soundpost is that it serves as sort of a pivot point for the bridge, which in turn vibrates the top plate, which moves air, makes sound, etc. You need more of this action for the lower frequencies, which is why the post needs to be pretty much under the G.
All I know is the theory, I've never played one of these, and I understand that getting it positioned just right is a bit of an art. But if you were to swap the strings around, you would definitely want to move the soundpost as well.