There are some pretty good whitepapers on AD-OD out there that may be able to help you. Remember, though, PDC/BDC is a very different bird than AD.
What are you trying to change to an OD Replica? The PDC? Can't do it. The PDC *must* be on a Master. The BDC *must* be on an OD Replica.
So, to wrap - you can have an OD Master + PDC on one box. You can have an OD Replica + BDC on a different box. You cannot have an OD Master + BDC on one box. Make sense?
In PDC/BDC (NT) terms, there was a, well Primary Domain Controller and then Backup Domain Controllers. This was absolved in AD, as that implemented a multi-master scheme, so there was not one specific primary.
Ideally, you'll have two servers, if this is for mission critical services, such as OD and PDC authentication. One will have both of your "Master" role services (OD Master + PDC) and one will have your "Secondary" role services (OD Replica + BDC), so if you were to lose one, you would still have the service available to your network.
Hope this helps!
Michael