Got this problem too, but only on the arm-bracer. the other stuff I can switch and buy, just not the bracer.
EDIT: Say guys, when any of you got this problem, did you buy any extra armor before or after you got the dual blade?
EDIT2: I figured it out; to prevent the armor problem only buy armor until after you have received the dual hidden blade from Leonardo. Or of course if the mission specifies you having to buy it. I do not know how one can fix this problem if you have it. only thing you can really do is start a new game and prevent it from happening again.