How Customer Sources Work

Source: `customer_sources_controller.rb`

A customer source's code cannot be modified if that source is already assigned to any customers; the code is effectively locked once in use
A customer source cannot be deleted if it is currently assigned to any customers; all customers must be reassigned to a different source first

"I can't edit this customer source code" → Customer source codes are locked once assigned to any customers. Create a new source with the desired code and reassign customers if needed.
"I can't delete this customer source" → The source is in use by one or more customers. Reassign those customers to a different source before deleting.