Sync Backup Directory Node (v 5.x) and sync with Primary Domain Controller (4.x)

instead to upgrade an exstiing UCS PDC + Backup + Slave from 4.9x to version 5.x, I want to setup a new clean DC infrastructure.
However, all existing AD data should be taken over.

That’s why I wanted to set up a backup server (backup directory node) version 5.x first, sync with an existing PDC (version 4.x - if it then takes place) and after switch off the old DC.
The apps (Kopano, Samba, Nextckoud etc.) which are currently running on their own members servers (4.x) should then be gradually taken over.

Does this work or can it lead to various problems and if so, which ?

no answer, no support ?

why not just take the 4.9->5.x
THEN add a clean 5.x backup and migrate it over.

I think the master has to be the LATER version, whereas you are making the master an older version than the backup.

so you cannot have slaves of a higher version than the master

seems like a mistake…