I can see that you're using the Desktop version, E21J. Let me guide you on how it works in that specific program.

You're unable to convert customers to jobs. You'll have to inactivate and recreate them. Then, set them up under a specific customer.

First, here's how you can inactivate a customer:

1. Click the Customers menu.
2. Go to Customer Centre.
3. Select the customer, then open the profile.
4. Tick the Customer is inactive box, then click OK. 

Then, here's how to recreate them as a job:

1. Click New Customer & Job, then select Add Job.
2. Enter a job name.
3. Click the CUSTOMER drop-down arrow, then select the parent customer.
4. Enter an opening balance if necessary, then add the date.
5. Fill in the necessary fields. 
6. Click OK. 

If you change your mind and wish to merge them instead, I'll share you the article with the steps to better guide you. 

Don't hesitate to drop a comment below if you have more questions. We're just here to address them.