I have an object with many fields that should be editable when a NEW record is being created. However, only 2 of these fields should be editable for an existing record. Other fields should not be editable for existing record. I thought I could use field-level security settings. But using that, the fields are not editable even at Create-time. How can I make a field to be editable at create-time (new record) but non-editable at edit-time (existing record).