Both the patient identification - internal and the patient account number have changed as a result of the merge. See Section 3.5.2, "Merging patient/person information," for a discussion of issues related to the implementation of merge messages.
ADT ADT Message Chapter
MSH Message Header 2
EVN Event Type 3
PID Patient Identification 3
[PD1] Additional Demographics 3
MRG Merge Information 3
ACK General Acknowledgment Chapter
MSH Message Header 2
MSA Message Acknowledgment 2
[ ERR ] Error 2