To do this, you will need to get your character to the level that your weapon will need you to be after you have it upgraded. Make sure the item is repaired or this will not work. After you do so, speak with the Artisan in the market to level your item and have it done. You will now be using the weapon without having the requirement that it asks for. Also if you unequip it without doing this, you will not be able to re-equip it again without gaining the requirement first.