I'm more surprised this wasn't already the case.

Deadheading pilots on AC are guaranteed J, and they will downgrade paid J to make space, even if the "deadhead booking" occurs at the last minute.

That being said, I've only heard of one case where someone was actually downgraded for a pilot, and they were not paid J. And I've never missed an upgrade on AC where there were any pilots in J.

I doubt this will have much of an effect.