Howto register an old cluster, formerly managed by an ancient Rancher version?

Hi folks,

I would like to register an existing Cluster (“kube001”, on-premises, setup using Rancher v2.2.8 or older) on my current Rancher setup (v2.5.1). AFAIU I have to disconnect kube001 from the old Rancher first, but how can I do that? Clicking on [Delete] is surely not an option. I don’t want to loose kube001, I just want the old Rancher to forget it and never bother about it again.

Do I have to cleanup kube001 somehow first, before registering it in the new Rancher? Sorry for asking, but the documentation doesn’t tell.

Every helpfull comment is highly appreciated