Does anyone know if it makes a difference if you have paper or electronic tickets?

I am trying to upgrade from C to F on an OZ flight (using UA miles) and I keep getting the message "The flight segments you requested to upgrade are not available to upgrade." I am not sure if this is because there is no u/g inventory available or something else.

I called UA.bomb support and the rep said that it could be that there is no inventory available or that it could be because I have a paper ticket "that can't be viewed in the system." (That seems odd, IMO.)

I have not seen any rules anywhere stating that only e-tix will work.

Regardless, I'll try everyday until flight time, and if still unsuccessful, I'll ask at the airport.