Rule 120.20 tickets are most certainly upgradable, unless you were interlined on a coach award ticket, in which case they might argue that the $0.00 base fare isn't giving them any revenue (which couldn't be further from the truth).
To avoid this problem, and security-related problems, always book your own reaccommodations yourself and have the carrier that is interlining you either print your ticket or write you a FIM (flight interrupt manifest). Hand one of those items + your confirmation # to the carrier that will transport you and confirm your upgrade. Also, it's better to do this inside rather than outside security to further reduce the possibility of unwanted haraSSSSment at the checkpoint.