Thanks Wiki.
I just spent 30 min on UA's website trying to figure out how to put in a mileage upgrade request for a single segment of a CLE - ORD- SAN flight. Then I remembered, "Hey! I can do more than just rant on OMNI/PR. I can actually use flyertalk to find out things about flying, like the good old days!"
Originally Posted by first page of the wiki
"You can request upgrade on a per segment basis (e.g., requesting only IAD-SFO on a XXX-IAD-SFO itinerary). Once the longer segment clears, you can request to extend the upgrade on the shorter segment. This can only be done over the phone with an agent."