You're right. It'll cost you 25K miles for each ticket. What dates are you travelling? You could check availabilities on aeroplan.com by clicking the 'Book Online' function near the top or let one of us know if you need help in checking.
If I recall correctly, if you are using your Mileage Plus points for AC awards, you can change your dates for free before departure. Once you start your trip, no changes can be made. You can only make changes to a Mileage Plus award after departure if it's UA, LH or TG.
[This message has been edited by Empress (edited 07-26-2002).]