The powers, at the moment, once purchased are permanent. The fighter can always of course discard an old item and begin on a new one. The only powers currently available are those listed (with good reason too I might add). Also while it is possible to buy the same power more than once for an item, any one signature style or location can only be purchased once. A holy warrior with a mastered style for instance cannot purchase his single style multiple times for the item giving him huge point additions.