A charge back would not be appropriate in this case if this were a revenue ticket; you'd only get a partial refund (unless full J in which case you can get a full refund regardless). AC has a policy on this. The link below is for international, there is also a tarrif document for domestic flights.

https://www.aircanada.com/content/da..._Tariff_en.pdf

Rule 30.A.1.a states that there is a surcharge of $315 that applies to lie-flat Signature Class seats (I assume the $315 is CAD).

Rule 100 D deals with carrier-caused refunds, including downgrade of service.

• Subsection 2.c.i states that you're entitled to fare difference, if applicable, if the change is made more than 7 days in advance.
• If the change is less than 7 days away, the table in subsection 2.c.ii would apply (note that Signature Class and Business Class are different classes on the table); in this case you'd get a refund of the $315 surcharge.

AFAIK for Aeroplan tickets, you redeemed a "business" class ticket. While you may have been seated in a better seat than typical business class seat, regular business is all you redeemed for. In the past IIRC AC has treated the lie-flat seats as a "bonus" for Aeroplan redemptions. You are not entitled to any refund (unless you paid a premium in the fare calculation as stated by AC*SE), but AC may offer you some goodwill discount for future travel. Aeroplan might also be lenient on change fees for such a situation.

Also, please note that things do change. Rule 100 D.d states "Refund can only be determined once travel is completed, and the appropriate class of service has been provided." You may want until after travel to complain as things could always change back; AC might be more willing to give goodwill compensation after the fact.