I had this happen to me once. Aeroplan refunded the points difference between economy and business for the distance portion of the downgraded flight. For example, if on a LGA-YUL-YVR routing the first segment is downgraded to all-Y aircraft, the refund in points will be 1,546 Aeroplan miles:

(25,000 - 12,500) * 324 / 2,619 = 1,546