Is there a good list that shows which UUs keep their special traits when upgraded? I understand that some of them do and some don't. Like the Chinese since the crossbow eventually upgrades to a rifleman I'm assuming you don't get a rifleman that fires twice. But what about some of the other civilizations? Does the Samurai keep it's bonus for producing GGs faster when upgraded?
Technically, the Chu-ko-nu does retain its special trait (Logistics) when upgraded to a Rifleman. However, Logistics (the double attack special trait of the Chu-ko-nu) is a promotion used by Ranged units, and since the Rifleman is NOT a Ranged unit, the upgraded Chu-ko-nu effectively loses its double attack.
There's not a list of which Unique Units pass their traits on when Upgraded, but the most important thing to realize about retaining Unique Unit special traits when Upgrading is to realize that the game treats retainable UU special traits the same as promotions.
This leads us to the basic rule of thumb for determining which UU special traits are retained, and which aren't.
A Unique Unit's special trait will be retained when the unit is upgraded IF AND ONLY IF the UU special trait is represented in one of those promotion icons (those little squares in the Unit Status window above Movement Points).
This, in turn, leads to more specific examples.
1. Boosts to numeric stats (Combat Strength, Ranged Strength, Movement) are not passed on when Upgraded. Example: Ballista has more Combat Strength than the Catapult it is based on. Upgrading a Ballista to a Trebuchet will not result in a stronger Trebuchet, because the Ballista's Combat Strength isn't represented as one of those promotion icons.
I'm pretty sure the Legion's ability to build and repair roads isn't retained, because that ability isn't represented as one of those promotion icons.
