Strictly speaking I am running Slowroll, however I understand that its packages are a subset of those in Tumbleweed.

https://en.opensuse.org/SDB:KDE_repositories gives procedure for switching Tumbleweed or Leap to use relevant repositories.  In both cases, adding repositories and executing "zypper -v dup --allow-vendor-change" completes.  The last command "zypper -v install pattern:kde_plasma6" fails, with a message that "patterns-kde-kde_plasma6" could not be provided by any available repository.

Package "patterns-kde-kde_plasma" is available, indeed installed. The nomenclature for it uses the convention being followed for other plasma6-related packages, namely suffix "6" is omitted. Packages relating to Plasma5 are suffixed "5".

Does the manifest in "pattern:kde_plasma6 need to be updated.

Result for Leap 1.56 beta is similar. I have not attempted to switch Leap 15.5 to Plasma6