Hi @Anonymous
Before demoting the DC 2003 , you should check the domain controller health and fix any problem in case of problem.
It seems that DNS zone replication was not completed correctly between DC 2003 and 2012.
It's important before any Domain controller demotion or promotion to check domain controller health.
If you can restore the first DC 2000 , you should relaunch the upgrade. If it's not the case I recommend you to build new domain and migrate all users and servers to it.
Please don't forget to mark helpful answer as accepted