I actually think it is pretty simple. Had my old Zentyl 4.2 PDC crash last night and started getting calls at 5:30AM.
The biggest problem is dhcp Failover, i have about 30 Reserved ip addresses (timeclocks, printers, NAS). All I had to do was log into the bdc remotely and enable the dhcp module. Withing 5 minutes everyone was back up and running.
I have not tried leaving both DHCP servers up and running, but from windows days, it doesn't seem like a smart idea. ( if using the same scope). I do know that if windows server detected another DHCP server on the network it would shutdown. Not sure about ubuntu and there are some DHCP failover tutorials for 14.04LTS, but i don't really want to start modding my pdc if I can help it. I was thinking of copying all reservations to both pdc and bdc then splitting the scope from a .100-.199 to a .100-.49 and .50-.199. Reservations will always get the same address from both servers and any domain computers would be able to authenticate to their server without issues. I might virtual it up and do a few tests if I ever get a few seconds.