Well, it's done, I downgraded. A wonderfully pleasant woman from Tesla (Naomi) called and said thanks for the email, that they've had numerous inquiries, and that the feedback has been conveyed to management, but that Signature pricing was pretty much set. She said if I downgraded that Tesla will do their very best to fill an empty slot in the general production line equivalent to where I would have been when I first made the reservation. In my case, that should be simple since the #2840 slot should be empty since that was mine before the upgrade. For someone that went straight to Sig, I'm assuming they'll find you an empty slot that's in roughly the right area for when you made the Sig reservation.
She did warn though that if I waited a few months and then decided to downgrade, that I'd be pushed farther back in line behind folks that had already made the downgrade decision, which seemed fair enough. Knowing that, I told her to go ahead and downgrade me now.
I'm sad to not be a Sig holder, but I'm happy that it looks like I'll get something close to my original spot in line.