@cjloveconsulting. Thanks for your reply. 

Question: Would scheduled automatic recurring payments be a cause?

These sub-clients do not have any actual unposted payments or pending payments other than possibly the above scenario?

It still doesn't 'compute' in my brain as to why we were able to change some of a client's sub-accounts from 'bill with parent' to 'bill with client' but not others. The main client pays all of them at the same time. When comparing one sub-client to the other - there are no differences. It makes no sense why we can change some and not the others under the same 'parent' client.