If your first Windows AD server is getting decommissioned, you need to transfer (or if the Windows server is offline) seize FSMO roles. Have a look at samba wiki: https://wiki.samba.org/index.php/Transferring_and_Seizing_FSMO_Roles
More info on FSMO roles: https://wiki.samba.org/index.php/Flexible_Single-Master_Operations_(FSMO)_Roles
Here is a procedure to change from Windows 2012AD to Samba4 AD: https://serverfault.com/questions/703724/migrate-from-ms-active-directory-to-samba4. I don’t know if the current version of Samba4 can handle a Win2012 AD domain? This topic is 3 years old.