Slitherine

I wanted to buy some trucks for my german ´37 engineers, but starting from the invasion of Norway, when you get ´39 engineers, you cant buy transport without upgrading the engineers first to ´39 engineers. Dosnt make much sense, i just wanted some trucks, dont understand why i have to also upgrade the unit first aswell.

I think you have to select the same unit to add a transport but if the unit is no longer available, this is no longer possible ?