Downgrading an upgraded segment requires your original fare class to be available, which become less likely the closer you get to departure. However day of travel it usually does become available but the remaining choice of economy seats can be limited.

Upgrades that don't clear at booking tend to sit and the next best chance is commonly fairly close to departure.

The best strategy for an AAA-BBB-CCC itin is to only request the upgrade instrument for the longer segment and only add the short segment when the longer clears.