First of all: I know I am doing something wrong, but just don't have a clue what it is.

I created 5 plans in CBSubs, but only activated one. It does not allow upgrades, but is published, does allow new subscriptions and allow registration is enabled. Still at index.php?option=com_comprofiler&task=pluginclass&plugin=cbpaidsubscriptions&do=displayplans the plan does not show, and the only way to get it there is allow an upgrade to it (which obviously makes it show as an upgrade).
There are currently no subscriptions whatsoever, and the plan is not a child plan.

As said before: I just don't have a clue, and am fully aware that I missed something. Can someone please offer some guidance?
Thanks in advance!

I assumed that a membership would mean that in this case an already registered user would apply for a membership, and that an upgrade would mean that there already must have an active (parent) CBSub subscription.
It seems you mean that even the registration itself is a membership, so every already registered user can only upgrade.

That is just different than I would have expected, and I can see the benefits of that.

Thanks for the fast response Nick!