Items in the game have one "Slot" for a Player alteraion, it is more relevent on armor, that can recive a armor kit or a enchant, in the case of weapons, it is almsot exclusivly for a enchant, placing a enchant or armor kit on a item uses that slot, and can be overwriten. Regardless, you cannot overwirte the Base Stats of any weapon, even if the stats look like a enchant like some items that process a fire strike on a chance hit, that are then enchanced with Firey weapon dose not loose the base magic of the item, it is only aided, when you try and add a new "Enchant" you will get a message about overwirting.