There's no record of the transaction with regard to mileageplus miles, which is the problem. I've been rebooked on the new flight, but there's been no change to reflect the fact that the new flight is fewer miles than the original one.

And, despite my language about redepositing the difference (this is how it's presented when you change a flight), you're right that the actual transaction that would take place would be a full redeposit for the cancelled flight and a rebooking to the new, cheaper flight, effectively resulting in the difference being redeposited.