This recipe makes use of a standard controller, so we only need to create the Visualforce page.

Opening the following URL in your browser displays the ContactCreateEdit page to create a new record: https://<instance>/apex/ContactCreateEdit.

Here, <instance> is the Salesforce instance specific to your organization, for example, na6.salesforce.com.

Note that the section heading for the page is Create Contact, and the Account Name field is editable. Save this record and edit it again via the same page using the following URL: https://<instance>/apex/ContactCreateEdit?id=<contact_id>.

Here, <contact_id> is the record ID of the newly created contact, and it displays the edit form of the page with a different section heading, and the Account Name field is changed to read-only.

The Visualforce page utilizes a standard controller and conditionally renders the section heading, and input/output variants of the Account Name field based on the ID of the record.

<apex:sectionHeader 
    title="{!IF (Contact.id==null, 'Create', 'Edit')} Contact" />

<apex:inputField value="{!Contact.AccountId}"
        rendered="{!null==Contact.AccountId}" />
<apex:outputField value="{!Contact.AccountId}" 
        rendered="{!null!=Contact.AccountId}" />