I have just done the same thing. I am on absolute 2 and had no idea you had to upgrade the whole absolute collection and can not just upgrade groove agent. I have tried to contact Steinberg however the support keeps kicking me out of my account.
It does not say anywhere on the website that you can not upgrade just groove agent, it literally only says it in the shopping basket but I am too busy trying to remember my username and password for the shop to even read it.
Did you get any help with this? Thanks