When you post to an account, you can no longer change the account number with the end-user versions of the program.  If you are working with an accountant or consultant with the Accountant's Edition of the program, they can change the account numbers for you even if the accounts already have entries posted to them.

Also, you cannot change account numbers if data entry screens are open at the same time.  Make sure all windows are closed using the button on the Toolbar of the Home Screen (also on the View menu).