I'm being told by a Aeroplan phone agent that I can't select seats other than the one's they assigned because "you are a 25K, but your travelling companion is not"

Full story:
1) In March, booked YYZ to PHX Economy Class (T) for November, returning also November. This was, at the time, a Rouge A319-100 flight
2) Booked on points, with $ for fees, taxes, etc.
3) was able, as part of the booking to select seats near front of Economy
So far, so good

4) On August 8, Air Canada informs me that they have changed the plane (and the times slightly). Now it's an Air Canada 787-9 flight (yes, different seat config)
5) New assigned seats are much less desirable
6) Go onto Air Canada site, look in My Bookings, select the flights, go through the process of picking the seats I want. (the seats I wanted all showed as free). At end of process, I get a “We're sorry, your new seat selection is no longer available or cannot be booked. Please make another selection.” message.
7) Try again (a couple of times, just to make sure it's not me making a mistake.) Nope, same result

8) On August 23, talked to an Air Canada/Aeroplan agent who tried to change the seats. She couldn't, and put me on hold for ~15 minutes to figure it out.
9) Eventually explains that she can't make the change because "while you are a 25K status member, your wife is not. I can move you, but not her."
10) I try to explain that the system certainly allowed me to choose the better seats (together) on the initial booking, but she just keeps going down the "I'm not allowed to change it" rathole

Does this make any sense? I've booked many flights over the years for the two of us, and never had a hint of what appears to be 'your seat selection ability will default to the status of lowest status person'

If I missed something fundamental, please let me know...