There were some posts a while back asking about SDC on a ticket issued by another airline, and someone replied that it wasn't allowed. Is this confirmed, and/or still the case?

I have a ticket issued by United, and would like to take an earlier flight from YYZ-EWR. The fare class is P (business class). Both the flight I'm on, and the flight I want to change to are operated by Air Canada. According to the Air Canada website, either the SDC or standby should be free for this fare and routing - doesn't say anything about who has to issue the ticket.

Can anyone confirm? Also, is it possible when checking in online or only at the airport?