All f.o.x. gear has a lock on them so you don't acidentlly use them to upgrade click on the lvl 1 ss rank f.o.x. gear you want to use to limit break you'r lvl45 f.o.x. and there should be a padlock icon at the bottom of the information window for it unlock it then go to you'r lvl 45 f.o.x. gear and go limit break and you should see the lvl 1 you unlocked