Of course, I have copies of K4PC going all the way back to 1.2.1, so it's not an issue for me personally, but for those who got caught this time, go grab a copy of 1.8.3, just in case.
I still have one of the original beta versions.
It can still be registered and used just fine. At least it seems they're fairly committed to supporting their legacy apps/devices. I realize that could change at any time, but I'd think if they were going to force someone to upgrade... they would have done it by now.