One question...have there been irrops in the past? Other (UA-caused) problems like schedule changes? Agents are willing to do a lot during irrops, including letting you handle getting to the next leg of your connection - especially for short flights (for example, PHL-EWR, EWR-IAD, etc)
You can certainly convert your ticket to a one-way and keep the remainder as a travel credit. Is this what has happened in the past?