I accidentally dismantled iconic jackets. I know I can craft new ones, but will I be able to upgrade them from rare to legendary, since I'm at level 50 and you need (from what I understand) to level them 12 times for that?
Yep, that's unfortunate if you are at level higher than 38, you're unable to upgrade 12 times to go to legendary
(you don't have a save before dismantling these items).
