IMO the result of the effort should be to be able to have multiple DC’s in the network and be able to move FSMO roles from the first DC to any other DC. This is not only to have redundant DC functionality, but also a strong point when you have to migrate to other hardware without a HUGE backup/restore effort of your user and devices on your network.
scenario: you have an old server with DC role. You get a new server, also give it DC role by joining the same domain. Then transfer FSMO roles to the new DC. Install all services on the 2nd server that were done by the first server and finally demote the first server as DC. So you can safely remove an old server from the network.