Don't skip (either flyer) the intial leg or the rest of your trip may be cancelled.

About the only way to drop the initial leg without paying a change fee is to have a sufficently large schedule change. At that point you probably would be able to get UA to drop the first leg. The change by rule needs to be 90 minute change in arrival or deaprture or a routing change adding an additional stop. Some have gone "free" changes for less then 90 minutes.