Readonly=true only while updating or hide some attribute while updating entity

I Want my Add/Edit screen different while creating New Entity and Updating Same Entity.
What is the best way to achieve? [May be explicitly set readonly=true before it show to screen in case of updating ]

Use InitEntityEvent for customize display of the screen when you create a new entity.
This event is triggered only when a new entity instance is created.
You can set your own flag in the handler, for example isNewMode. Check flag and then hide the necessary attributes in onBeforeShow .