Create a parallel infrastructure based on Linux Servers with Samba4 (Figure #1) and join them to the Windows domain, this leads to the replication of Active Directory objects between the two platforms, the process can take some time depending on the amount of information contained. Then start SysVol replication manually, because Samba AD currently doesn't provide support for SysVol replication is necessary use a tool to sync the content, for example rsync (support replication in one way). Finally transfer the FMSO Role from Windows DC to Samba4 DC and demote a Windows DC. The process is quick and claims to be completely transparent to the user because it uses replication to maintain the integrity of the information from AD. As you can see in this case you must follow the steps explained in
Join Samba4 as additional Domian Controller.