Can't transfer user folders/files from local user to SAML user

Nextcloud version (eg, 12.0.2): 15.0.0
Operating system and version (eg, Ubuntu 17.04): Debian (latest official Docker container)
Apache or nginx version (eg, Apache 2.4.25): 2.4.25
PHP version (eg, 7.1): 7.2.13

The issue you are facing:
We want to use SAML with our Keycloak server but the web form login does not work in any desktop client (2.3.3, 2.5.1 etc.) because of
Therefore we would like to use local Nextcloud users until the bug is fixed then move their ownership to the SAML users via occ files:transfer-ownership. However that does not work either because of this bug:

The following however does work:

  1. Create a local user with username testuser and e-mail:
  2. Login as that user, upload some files
  3. Create a SAML user in Keycloak with username testuser and e-mail:
  4. Enable SAML, login in the web interface or with one of the mobile apps (those are not affected by issue 279)
    Result: The uploads from the local user are there

So all that would be left is getting rid of the local user after we switched to SAML completely but once you delete testuser, enable SAML again, login as testuser (from SAML) all the files are gone as well.
Is there a way to get rid of that local user while still keeping the data so we can switch to the SAML user instead?

Or is there even help for any of the original bugs listed above?