By default the higher family flare code wants to be the domain controller.  You will need to make one of the CX4's the domain controller.  If you can bring the CX4's into the domain and they say unsupported next to them then promote one to domain master you will retain the accounts.
Hello,  I have an existing CX3 domain (Flare 26) and I want to add CX4s with FLARE 29. The existing domain doesn't support the newer FLARE. I could just create a new domain, but there are user accounts CX3 that are used for scripts and for which I don't have the password. Users and passwords on the CX4s can be discarded. Is there any way to get all boxes together in the same domain, saving existing user accounts and passwords?  Thank you!
How to add new Clariions to an existing domain