I have a picklist on a custom object called Status__c.  This field has many value options including "Assigned" and "Sent for Revision". If the user is a particular profile, I ONLY want them to be able to change the picklist value if the current value is "Assigned" or "Sent for Revision".  So far this is what I have:
AND(ISCHANGED(Status__c),
OR(
NOT(
 ISPICKVAL( PRIORVALUE (Status__c), "Assigned"),
 ISPICKVAL( PRIORVALUE (Status__c), "Sent for Revisions"),
 ($Profile.Name = "Freelance Writer")
)

Of course this isnt correct, but Im having trouble determining where my logic and/or syntax is off.
 

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).