Having the same problem. Import already existing clients is not possiblie (nor via powershell, not even with parameter "mergeifexist" neither via GUI of CM Client).
We side-by-side migrated from old CM-Env. to new and also migrated all collections. As workaround we are using now password in PXE + OSDComputerName.
We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.