Do the LT WT ratings for railcars include the truck weight?
The Light Weight of a car is everything on the complete car - trucks, brakes and appurtenances fitted to it (auto rack spring to mind).
Does that mean that, if the trucks are replaced with a different make/model (assuming that those from different manufacturers have different weights), that the car has to be re-weighed and the markings have to be changed?
Yes
That also explains why scale test cars don’t have brakes. The weight of the brake shoes would change with every brake application.