My understanding -- which should be taken with two grains of salt, I'm a programmer not a doctor -- is that the pacemaker cells which actually go "beat now beat now beat now" are part of the heart itself, and transplanted with it: https://en.wikipedia.org/wiki/Cardiac_pacemaker
The nerves from the rest of the body are needed to tell these cells "we need more oxygen, beat faster" or "sleepy time, beat slower".
I assume if they could reconnect the nerves to the new heart they would. There's no need to replace the pacemaker cells in the new heart with the old ones though.
The nerves from the rest of the body are needed to tell these cells "we need more oxygen, beat faster" or "sleepy time, beat slower".