In my experience, the policy is that "you're on your own"

Basically, the automated rebooking system will rebook you in your original class of service, for better or for worse--at least, it does rebook you. But neither confirmed upgrades nor upgrade requests will carry forward.

However, whether I talked to reservation agents, RCC staff, or customer service agents, UA has always ben willing to reapply, and even force upgrades in those situations. One of the most striking examples is when my SFO/HKG flight was cancelled, and IM opened up a NC seat for me on the next flight. (I was confirmed in NC on the original flight.)

I wish the system did know how to treat upgrades--it should at least waitlist folks. But I'm very happy with the service I've always received in those situations. RCC access (as well as access to the 1K line) do certainly help.