In general I'd check that domain replication and health (repadmin /dcdiag) are 100% before making changes. No changes needed to replication. Only change should be to windows time services. Change new PDC to use an NTP source and other one to use NT5DS source.
(please don't forget to mark helpful replies as answer)
Regards, Dave Patrick ....
Microsoft Certified Professional
Microsoft MVP [Windows Server] Datacenter Management
Disclaimer: This posting is provided "AS IS" with no warranties or guarantees, and confers no rights.