Well, I'd think that the people that tend to believe in the power of astrology are the sorts of people that think that everything happens for a reason. If that's the case, then they'd say that you were born early for a reason (whatever that may be), and hence your "intended" birthdate is your real birthdate not your due date, and hence the astrologically relevant dates.
And I've probably just written a load of rubbish, but hey. For me, my due date was only 5 days after my real one, so no star-sign change anyway. Not that I believe in astrology lol.