I added a State property to each structure.
I created a Display Rule that reads the State and if it's "Existing" it sets the elements' Transparency to 80%.
Note that this capability is universal, we can use on any Feature Type. I'm using an Item Type on a Feature (not a drainage UDX)
right-click to save: test-drainage-transparency.dgn
Item Type definition is in the dgn as is the Existing, Design, etc. picklist.
There is a bug in the automatic rescan of the Display Rule. In order for the view to refresh properly, the utility database needs to be rescanned. The fastest way to do that is to switch to a different Display Style that uses a Display Rule that reads the utility database (say, Drainage is active), then switch back.
Multiple Features - this works on any Feature (drainage or otherwise).