I canceled a non-refundable JFK-SFO roundtrip in C last year that was originally ~$2,000. The next itinerary I purchased was about $800, so I used the funds from the canceled trip and was automatically sent a voucher for the remaining $1,100 ($2000 - $800 - $100 change fee).

I then used the $1,100 for another itinerary worth about $600. Since it was a paper voucher, I did ticket-by-mail. I was then automatically sent another voucher for $500.

You can rinse and repeat the last step over and over again until you've used up the balance. As luck would have it, I ultimately ended up with a voucher being sent me to me that was worth less than the stamp United used to mail it.

As far as your question of using the credit for a web fare -- the only way I can imagine that would work is if you could hold the itinerary at united.com and then call in to have it ticketed, but I'm not sure even that would work.