different fields for add and change pages in admin

This is an old question but I wanted to add that the add_view and change_view methods can be modified for this purpose:

class SoftwareVersionAdmin(ModelAdmin):
     ...
     def add_view(self,request,extra_content=None):
         self.exclude = ('product','version_number',)
         return super(SoftwareVersionAdmin,self).add_view(request)

     def change_view(self,request,object_id,extra_content=None):
         self.exclude = ('product','description',)
         return super(SoftwareVersionAdmin,self).change_view(request,object_id)