I'm not aware that UA (also pmCO) have a DL-like program with the UP fares. The only way to get upgraded immediately on a Y/B/M fare is to have the appropriate premier level. 1K/Plat/Gold/Silver for Y/B and 1K on M fares.
If you search and purchase a first class ticket on UA you are buying into the bucket directly and not going through Y/B/M first.
However, like the DL system, if you do use the immediate upgrade on the Y/B/M fare based on your premier status and later on there are flight issues (plane changes, etc. etc.) you are only entitled to a coach seat but you'll have very high priority for the upgrade at the gate.
-RM