Based on my experience, NO.
On one flight AC015, when there was an equipment change resulting in a loss
of J seats, my wife was on a Y fare with upgrade confirmed months before.
At check-in, they told us her upgrade was revoked because she did not
have status, even through she was my companion, and would only guarantee
a J class seat if we paid the fare differential to J.

That led to a long argument with concierge as to what a "confirmed upgrade"
at time of booking meant, and eventually she got it; but my understanding
of the way they did things was that other members with status would
have gotten the UG ahead of her, based on her own status.