From
Aeroplan.com:
If a change results in a decrease in Aeroplan Miles required for the reward travel, the difference in miles will be forfeited by the member.
To receive the difference in miles, the member must cancel the existing reward travel and start the reward reservation process over again. The member is subject to all associated fees and potential loss of reward seat availability.
So say someone redeems 120K for a First Class Star Award to "Asia 1", come two weeks before departure, the passenger simply cannot go. They pay the $55 fee to 'cancel', but they can't get the miles back.
But the 120K is 'locked up', it can't be redeposited and if the miles are used for an award that's less than 120K, the rest is forfeited... so if the ticket is rebooked for say Business Class to Europe 1, 40K miles are forfeited... one can only 'pay up' by throwing in more miles, but no miles are going to be returned...
That is rather scary... takes away even more flexibility than the new change fees themselves.