Thanks for your response.  I have tried that and I receive an error that says this:

"You cannot change who this client bills with because there are invoices for the parent that include charges for this client, or there are payments from the parent that pay off invoices for this client. Delete all payments or invoices to this client's parent which link to this client."

But this error makes no sense to me because there is a sub-client of a parent, assuming I have created an invoice for the sub-client then there will always be "invoices to this client's parent which link to this client".

Can I only uncheck this box once all sub-client invoices have been paid and none are outstanding?