I believe a cash refund may be possible, but who knows exactly how much. It wont be the full value of YTZDXB J vs Y, but probably some kind of prorated refund.

Well, that's how it would work for a cash booking, but as I've been showed here before, award tickets tend to be very different.

Curious that AC couldn't put you in J. It is EK so there might be some weird thing where they have no agreement for it. I know AC does sell BA J though so unsure why that couldn't be an option.

From my personal experience, AC will try to keep you on AC over the water (cost?), but if another airline gets you there significantly faster I doubt it would be refused.

Did you ask for that specific BA option and was told no? Or was it an after the fact thing you realized?