Hello @Bruce Ringler ,
Thank you for posting here.
We suggest we do not change the IP address and name of DC directly. However, if you just rename one, or the previous DC name will not be used again, you can also rename DC. Renaming the DC directly may cause risks.
For more information about rename DC directly, you can refer to the link below.
Rename a Domain Controller Using Netdom
You can use this procedure to update the File Replication Service (FRS) or Distributed File System (DFS) Replication member object after you rename a domain controller. This object must be updated with the new domain controller name so that the domain controller can replicate SYSVOL.
Update the FRS or DFS Replication Member Object
As you mentioned, the way below is suggested steps.
1.Take one of the 6 stand-alone servers 2019, IP and Name this server, than promote it to a PDC
2.Then at the PDC I want to rename and re-IP, remove roles and feature so it become a stand alone server.
Assume 2019 PDC : IP address 1, DC name1
Another stand-alone server 2016 : IP address 2, DC name2
1.Add a stand-alone server 2016 to the domain.
2.Add AD DS and DNS roles.
3.Promote it as DC (also GC).
4.Transfer FSMO roles from PDC 2019 to new DC 2016. Now the 2016 DC is PDC.
5.If new DC works fine and AD replication works properly.
6.Demote the DC 2019 to member server.
7.Disjoin this 2019 DC from the domain.
8.Change the IP address of 2019 server from IP address 1, DC name1 to IP address 3, DC name3 you want.
9.Readd this 2019 server to domain and promote is as DC (also GC).
10.Transfer FSMO roles from DC 2016 to DC 2019 again.
11.Demote DC 2016 if needed.
Tip: In one domain, having multiple DC's is a best practice standard. You should always have at least 2 DC's to safe guard against one going down. If all you had was one, and it went down, there'd be nothing for your objects to authenticate to. It's very common for environments to have more than 2 DC's as well.
Hope the information above is helpful.
Should you have any question or concern, please feel free to let us know.