The ISN has a great and easy function that allows to quickly merge agencies!
Navigate to Clients/Agents--> Agencies, and from there click on Merge Agencies:
Enter the name of the Agency and click Go!
The names matching what was entered will appear. Click the the boxes of the agencies that are to be merged, and then click Next:
Select the values that you want to appear in the final merged agency, then click Merge Agencies:
The result will then appear on the following page: