http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=064&top=10
Canada’s entry requirements are changing
Permanent residents (PR) of Canada must carry and present their valid PR card or permanent resident travel document (PRTD) when boarding a flight to Canada, or travelling to Canada on any other commercial carrier. If you do not carry your PR card or PRTD, you may not be able to board your flight, train, bus or boat to Canada.
It is your responsibility to ensure that your PR card is still valid when you return from travel outside Canada, and to apply for a new PR card when your current card expires.
Returning by private vehicle
There are
other documents you can use to enter the country.
When you return to Canada,
apply for a PR card if you plan to travel outside Canada again.
Some examples of private vehicles include, but are not limited to: a car, truck, motorcycle, or recreational vehicle that you own, borrow, or rent. It is a vehicle not available for public use.
They do not specify that you can use your COPR letter. I have to do some more research about it. Does anyone had similar situation or knows something more?