Thank you all very much for your responses and help.
As matters stand, I have lost the business class upgrade seat I wanted.
If a further seat become available, I will try asking for a "cancellation" with application of the remaining value to a new ticket in "exchange" and make it clear that I am willing to bear the cost differential.
However, it remains that the two agents and supervisor told me that I could not cancel and take a credit. Which leaves me disappointed and angry for having been misled and not properly advised. In this respect, I am making a further written complaint to United.
FINAL QUESTION: Is there any problem with my giving United a link to this thread?