I think there are two different things that are being conflated here:

Confirmed same day changes will put you in the same class of service.

If you can't confirm a same day change, you can use same-day standby. If you have to stand by, you are probably SOL because, as you noted (and is mentioned in the thread's wiki) upgrades are processed ahead of standby. But that's been true for a long time (that thread's wiki is not new).

I'm not sure what the agent was referring to. Maybe there is a reduction of whatever space/black magic is needed for SDFC in J?

By the way, on other airlines, I've often bought much cheaper flights than I've wanted with the intention of making sand day changes. AS allows same day changes to co-terminals, so I would often buy cheap tickets from SFO to LAX and then change to SNA, because flights to SNA were 3-4x the cost. So this definitely happens, though I can't comment on how pervasive it is.