Thank you for passing along some additional information. I was able to confirm that JetBlue provides same-day changes for a charge of $75 with no re-fare, and that the $75 fee is waived for Mosaic members.

Can you point me to a reference, because I had difficulty finding this “free change” policy on JetBlue’s website. Feel free to email me at scott@travelcodex.com

Likewise, I’m not aware of a free same day change for Southwest. A free standby does exist but is not the same thing and is common at most airlines.

For JetBlue’s same day change, it books in to any open Y seat. There is NEVER a fare difference charged for a same day change. A regular flyer pays $75 (no fare difference) and a Mosaic flyer the $75 is waived. Also Southwest does offer some kind of same day standby benefit with no fare difference but it is not as generous as B6 or AS.
But having no change fees is huge! If I need to cancel a trip with JetBlue, its no big deal. But even if I were 1K or Exec Plat technically I would be out $200 change fees to cancel a trip!
So in this way both B6 and AS do offer benefits not even rivaled by the big carriers!

“Free” same day changes on Southwest and JetBlue aren’t really free, in my view. Both carriers require you to pay the difference in fare, which can be substantial because you’re paying for a last-minute walk up ticket. Contrast that with the other four airlines that charge nothing. For changes in advance, not same day, then Alaska, JetBlue, and Southwest are comparable.

I’ve discussed same-day changes in an older post that is now a bit out-of-date but still speaks to this point: https://travelcodex.com/the-complete-guide-to-same-day-change-and-standby-rules/

But, as I said at the start, this post is not really meant to provide a comparison of JetBlue and Southwest.

Yes, I had that and then seem to have overwritten it with something else. United 1K and American EXP members both get this benefit. And I believe I erred, with Delta, which does not give this to their Diamond and Platinum members.