The sooner you book, the more protected you'll be. Since Cdn+ won't exist at the end of the year, the validity of your ticket will be based on the date it is issued. The major problem will come if you want to cancel your award ticket and redeposit the points, AC may not take them back into your new/existing Aeroplan account after Dec31st as they were Cdn+ points, and that program and those points no longer exist.
But if they confirm and ticket your initial routing on Cdn metal, and you make no subsequent changes, and CP no longer flies their metal into that city, AC would have to either recredit the points into your account, offer you an alternative but equivilent ticket on a route CP does fly, or protect you on their metal/or that of a STAR partner.
In any of these cases, I would not worry unless you let your certificates expire in October. I booked my mother on a Cdn Business Class award earlier this year. With all the sked changes, Cdn rebooked her in Executive Class on AC even though this occured prior to the date they started reciprocal awards.
The critical caveat to all I have said, and a very important one, is this: if your awards are Amex discounted ones, they can only be flown on CP metal and you would be asked to "pay" the difference if you had to fly AC metal on any segments (unless it was AC/CP who rebooked you under protected status).
Deos all that make sense? Hope it's of some help to you.