For what it's worth, I took my car in for the upgrade yesterday, the day the price changed. I'm picking it up today and I asked whether I would get the new price. The service advisor replied that since I was still in service, I would get the new price. This indicates to me that if you've paid and driven away, they are not going to do anything for you. In a normal sales environment, most companies would offer you the same discount if you were a within a couple of days, but I guess where to draw the line is the dilemma....
Also, they seem to be automatically upgrading the driving computer to HW3 as well, so you get that even if you haven't purchased FSD. The part number for the replacement part indicates Intel/HW3 together on one line and the tech confirmed it to me when I dropped it off.