The ISN has a great and easy function that allows to quickly merge agents!
Navigate to Clients/Agents -- Agents -- Agents, and from there click on Merge Agents:
From there, type in the name of the agent you want to merge, and all agents with that name you select will appear:
Then, click the fields that you want to keep! You can click any field from any agent.
The information at the bottom of the page will then show the items you have selected to merge. From there click on Merge Agents Now: