I'm new to Salesforce and I have to allow edit permission to some fields only if they are blank. Otherwise they shouldn't be editable. My validation rule look like this for the moment, I'm a bit stuck. Probably I have to include ISCHANGED as well, but don't know exactly how
AND($Setup.Application_Control__c.Run_Validation_Rules__c = TRUE,
AND(NOT(ISBLANK(dfsle__CanManageAccount__c) || ISBLANK(status) || ISBLANK(username) )))