Ex: The Ceremonial Chest Scale and the Ceremonial Shield are techable unique items. When you've put a tech on one of these two equipments but later think you've made an error and would like to replace it by another one, it seems you can't.
There are several solutions:
1) A tech could be replaced on a unique equipment by dragging another one (the first tech is destroyed).
2) (The best according to me) Instead of giving a unique equipment, a formula is given at the end of the questline. This equipment is immediately attuned on creation (as in my example it is required to finish a questline to acquire it), so that it can't be equipped by someone that hasn't finished it (like Drain Bolt formula...I think, I don't remember whether it's attuned on creation or not).