Over the years I've had a couple of conversations with check-in agents about non-status flyers attempting to use upgrade certs. In both of these conversations (one as recently as a couple of weeks ago in YOW), the agent said that it ususally happens when a status customer passes on a cert without fully understanding the rules. The agents told me they simply tell the person presenting it that they aren't entitled to use it.

There was nothing in the conversations that suggested that agents would get shirty about it; in fact, quite the opposite. It seemed to be something that happens from time to time and that it can be handled without jumping all over the customer or confiscating the cert.