I don't think this can be implemented with today system. NQ and HQ items are different items from each other, the +1 its not a attribute on the same type of item. Thats said the problem relays in that if they want to give you a HQ item for a NQ version they need to take away the one you have a give you a new item, and if the equip have materias they have to transfer them to the new item.

Its not impossible but its messy enough to make SE say {No, Thanks you}.