does anyone know if it is still possible to upgrade to Pro for Cubase 12? …or is it absolutely not for sale any longer?
I think that you are trying to move a bit too late : only Cubase 13 range is for sale now, AFAIK, so the upgrade to the Pro version will cost you a little more than a strict Artist 12 → Pro 12 one could have.
But once done, nothing prevents you to download and install the Pro 12 version, if you really need only this one, either with the Steinberg Download Assistant or here.
Thanks for your feedback