QB Issue Resolution:

left side menu customers, click on a customer, upper right click the edit button or pencil

on the right side is a check mark box, sub customer of, check that then select the parent customer

Resolution for Issue 'I know how to create a sub-customer, but I need to know how to change an existing customer with many invoices into a sub-customer.' available: Yes (Solved).
Source: Intuit Community forum.

I know how to create a sub-customer, but I need to know how to change an existing customer with many invoices into a sub-customer.: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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