what the agent told you is essentially correct.

Redeposit the miles and start over.

However, a few of us have had good experience just showing up if all you want is a date change (earlier not later). Many partners seem to honor it at checkin but they nor Aeroplan will change it over the phone.

If you need a confirmed date and have no flexibility, it likely will mean redeposit the miles and rebook (if reward seats are available--the closer to the date, the less likely you may find them). The rules are quite clear for Star Alliance/partner awards but it doesn't make it any easier to take.

Good luck!