so if i understand correctly, they basically get enchantment that they already want in the first place right? since they can order it base on their desire or suitable talents, the chance they change their mind is unlikely especially regarding melee weapon that they are used to or master with and they choose that certain enchantment for a reason. it also can take a while to train your body to get used or accustom with this new enchantment.
sure you may encounter someone that is a bad chemistry with or has advantage against your martial style or enchantment or unit formations/compositions and force you to use another magic enchantment, but as @JBH has mention its far better to enchant blank slot weapon than rewrite the magic of the original weapon, and you may want to use back your old enchantment back later against the others, also now you have two weapon that can work as backup weapons or combine their magic enchantment with.
to explain better why i say you likely need to train your body to get use to the new enchantment of your weapon, imagine you originally use fire sword you likely know the safety movement to make sure the fire or heat dont reach or blow over your face or use proper armor for it and your body likely get use to the heat, outside of knowing how you can control your movement to use the fire to damage your opponent, to fast and you may lose the heat, but still it fitting with/become your martial style. and then you change to electricity, you need to learn a new safety to make sure you dont get electrocute yourself, it likely different than your previous one and require fitting armor against such element too. or if the enchantment boost your physical ability such as speed or strength or maybe make the weapon heavier or lighter, you need to train your body control and timing or perception to get use to that, especially if it boost your speed or manipulate the weight, otherwise it may ruin your balance or stance or you may accidentally overswing that can make vulnerable opening to yourself or make your weapon stuck to deep to pull out, or weaker swing that can result in less lethal damage or not deep enough cut to your opponent, because how light or how heavier to you, outside of your embedded muscle memory or habit not fit for this certain enchantment that can end up injure you.
after all we are talking about weapon here, its not just costly economically to do so, but it also can cost your life or your pride (if we talk about nonlethal duel) and time.