Account Number field is not displayed on Customer>Create

Sometimes we need to create a customer with Automatic Customer Numbering DISABLED... but when I disable (uncheck) it in System Options -> Trans and Customers tab  and go back to Customer page-> Create - I do not see the Account Number field to enter into (like if I did not uncheck that AutoNumbering flag) . What can cause it? Anything else needs to be done to propagate that flag value change in order to display the Customer web page right - to show account number field on the page? Should I every time when that flag change done to  'clear Cache'?