Well if that person is not really for me then I doubt I would be with that person in the first place. By this I mean that the entire thing would probably collapse even before it comes to something life this.
On the other hand if you are moving away and you know that the person can't come with you that means that you have already dumped the person. (technically speaking)
And if they are going away and you can't come that means you got dumped.
What would mean that there was no "madly in love" factor in the first place.
But I have never been in a relationship so I not in a position to say anything about this actually.