I got it from JRR and after ordering it says:
This license will be manually generated on your account page: https://www.jrrshop.com/sales/order/history/
However, if I go there, I don't see anything. If I go to the order again, I won't see the serial for registering the upgrade.
Edit: Nevermind, now it arrived. That also answers the question from someone here. You get an upgrade serial, you can register at bitwig.com at a later point in time. At least that's how it worked before.