Since you overwrote the old “DC” with the AD infrastructure, it is very hard to “reverse” unless you go back to tape or so (not recommended). I would have performed the actual demotion and removal of the server you wanted to replace. By simply deleting the server from AD simply would not remove other components of the environment. I would have performed a dcpromo of the new server utilizing a new name and then transfer all FSMO roles onto this server from those servers you plan to decommission. Just realize you need to be careful if you remove a domain controller that has the FSMO roles which are critical to Active Directory to function properly.
You indicated to prepared the domain but did you prepare the forest (forestprep) which is normally the first step prior to domainprep?
You may want to execute from command line the utility “dcdiag” from each DC remaining even on the new one.
1) Perform the forestprep if not performed
2) Confirm the new DC has all the FSMO roles prior to demoting/removing the old DCs.