Переопределите save_model
-метод вашего modeladmin:
class ModelAdmin(admin.ModelAdmin):
def save_model(self, request, obj, form, change):
user = request.user
instance = form.save(commit=False)
if not change: # new object
instance.status = ....
else: # updated old object
instance.status = ...
instance.save()
form.save_m2m()
return instance