[@CollectorOfMyst] it can be equipped immediately, and once it's equipped the upgrade can be removed at will to trade, sell, ect.

Upgrades that have a cost assosciated with equipping them, use up the resources that you equip them with. 

Forged Darkstone items, e.g at the blacksmith, are permanent upgrades to an item, since you've physically changing the composition of that item.

EDIT: Missed your posts so catching up with you, think we're on the same speed now :D serves me right for not double checking the thread before I posted~