The CRM application does not allow records such as orders, opportunities and invoices that are completed to be editied.
Once an order, for example, is marked as fulfilled it is read-only and the status cannot be changed in the web application.
However, the status can be changed via workflow as described in this CRM team blog:
http://blogs.msdn.com/crm/archive/2009/09/02/updating-a-microsoft-dynami…