Today, I changed both of the following, online, and they priced correctly and everything worked:
1. YYZ-ORD-MUC-NCE (with PR) to YYZ-EWR-NCE
2. SEA-YVR (with PR) to EWR-NCE
I don't know if there have been any very recent changes to the change system, but I think this is the first time I've been able to self-serve change a booking with a PR.
If even 50% (instead of 0%) of my bookings can now be changed online, that's a huge win for both me and everyone who actually needs to call in and wait on hold.