I do not work for Steinberg, but because of all the questions folks have had about the update/upgrade process (and the admittedly pretty complex update process), I ended up writing a step-by-step walkthrough, with screenshots, for the entire thing. While my steps cover purchasing the upgrade, there is a paragraph about how to check if you’re eligible for the grace period update, and what to do in that case. I have performed the update twice (Pro 11 → Pro 12 and Artist 7.5 → Artist 12). Most of the screenshots are from when I updated Artist.
I didn’t experience any problems yet, and I am still actively using Cubase Pro 11 on my main PC; I only installed 12 out of curiosity so far.