CO Gold for preferred seating but credit miles to Aeroplan?
 

Is it possible to change my FF program preference to credit miles to Aeroplan and still keep my preferred seating as a CO Gold? If so, when is the best time to do it- OLCI or at the airport?

Once the seat is assigned you can change the FF number at any time and not lose the seat.

If you get the EUA and try to change it afterwards you might lose the upgrade (you're supposed to but there may be some ways around it).

I noted that I added myself to the upgrade list and then changed my FF# to Aeroplan, I was still on the list. Of course, I was #8 on a full flight SFO > EWR so there was no chance I would have actually been upgraded, but still...