Yes, ELM33. You can make the existing client a job for another client without losing their transaction history.

To convert an existing customer to a job, you need to do it from the Add/Edit Multiple Customer window. Here are the steps to guide you through:

1. Open the Customer Center. 
2. Locate and right-click the customer name you want to change.
3. Choose Add/Edit Multiple Customer: Jobs.
4. Go to the Customize Columns button.
5. In the Available Columns section, select Job of: and click Add.
6. Place the Job of: item below the Name by pressing Move Up continuously, then click Ok.
   
7. Look for the customer you want to alter and pick the new Parent customer to connect it to in the Job Of: column.
   
8. Select Save Changes. 

Please note that if the customer has existing transactions, those transactions will now be linked to the new parent customer as part of the job. Refer to the screenshots for guidance.

If you need further assistance, feel free to ask. I'm here to help.