Amazon will refund the difference after the end of release day, in case the price drops anymore. Since the movie doesn't technically come out till Tues, it should happen then or Wed morning. It always has happened that way for me in the past.
You'll get the refund (without having to ask), they're always great about it. I just got mine today. The delay happens if the price drops after it enters the "preparing to ship" mode (in which case it requires a refund, rather than a simple reduction in the amount you're charged, and you'd only get the refund after it ships).